feat(agents): add Rust language support (#523)

* feat(agents): add Rust language support — reviewer, build resolver, patterns, and testing

Add Rust-specific agents and skills following the established Go/Kotlin pattern:
- agents/rust-reviewer.md: ownership, lifetimes, unsafe audit, clippy, error handling
- agents/rust-build-resolver.md: cargo build errors, borrow checker, dependency resolution
- skills/rust-patterns/SKILL.md: idiomatic Rust patterns and best practices
- skills/rust-testing/SKILL.md: TDD, unit/integration/async/property-based testing

* fix(agents): correct Rust examples for accuracy and consistency

- unsafe fn: add inner unsafe {} block for Rust 2024 edition compliance
- edition: update from 2021 to 2024 as current default
- rstest: add missing fixture import
- mockall: add missing predicate::eq import
- concurrency: use sync_channel (bounded) and expect() over unwrap()
  to align with rust-reviewer's HIGH-priority review checks

* fix(skills): correct compilation issues in Rust code examples

- collect: add .copied() for &str iterator into String
- tokio import: remove unused sleep, keep Duration
- async test: add missing Duration import

* fix(skills): move --no-fail-fast before test-binary args

--no-fail-fast is a Cargo option, not a test binary flag.
Placing it after -- forwards it to the test harness where it is
unrecognized.

* fix(agents): distinguish missing cargo-audit from real audit failures

Check if cargo-audit is installed before running it, so actual
vulnerability findings are not suppressed by the fallback message.

* fix: address automated review findings across all Rust files

- build-resolver: prefer scoped cargo update over full refresh
- testing: add Cargo.toml bench config with harness = false for criterion
- testing: condense TDD example to stay under 500-line limit
- patterns: use expect() over unwrap() on JoinHandle for consistency
- patterns: add explicit lifetime to unsafe FFI return reference
- reviewer: replace misleading "string interpolation" with concrete alternatives

* fix: align with CONTRIBUTING.md conventions

- skills: rename "When to Activate" to "When to Use" per template
- reviewer: add cargo check gate before starting review

* fix(agents): guard cargo-audit and cargo-deny with availability checks

Match the pattern used in rust-build-resolver to avoid command-not-found
errors when optional tools are not installed.

* fix: address second round of automated review findings

- testing: split TDD example into separate code blocks to avoid
  duplicate fn definition in single block
- build-resolver/reviewer: use if/then/else instead of && ... ||
  chaining for cargo-audit/deny to avoid masking real failures
- build-resolver: add MSRV caveat to edition upgrade guidance

* feat: add Rust slash commands for build, review, and test

Add commands/rust-build.md, commands/rust-review.md, and
commands/rust-test.md to provide consistent user entrypoints
matching the existing Go and Kotlin command patterns.

* fix(commands): improve rust-build accuracy and tone

- Restructure-first borrow fix example instead of clone-first
- Realistic cargo test output format (per-test lines, not per-file)
- Align "Parse Errors" step with actual resolver behavior
- Prefer restructuring over cloning in common errors table

* fix: address cubic-dev-ai review findings on commands

- Gate review on all automated checks, not just cargo check
- Use git diff HEAD~1 / git diff main...HEAD for PR file selection
- Fix #[must_use] guidance: Result is already must_use by type
- Remove error-masking fallback on cargo tree --duplicates

* fix: address remaining review findings across all bots

- Add rust-reviewer and rust-build-resolver to AGENTS.md registry
- Update agent count from 16 to 18
- Mark parse_config doctest as no_run (body is todo!())
- Add "How It Works" section to both Rust skills
- Replace cargo install with taiki-e/install-action in CI snippet
- Trim tarpaulin section to stay under 500-line limit

* fix(agents): align rust-reviewer invocation with command spec

- Use git diff HEAD~1 / main...HEAD instead of bare git diff
- Add cargo test as explicit step before review begins

* fix(skills): address cubic review on patterns and testing

- Remove Tokio-specific language from How It Works summary
- Add cargo-llvm-cov install note in coverage section
- Revert no_run on doctest examples (illustrative code, not compiled)

* fix(skills): use expect on thread join for consistency

Replace handle.join().unwrap() with .expect("worker thread panicked")
to match the .expect("mutex poisoned") pattern used above.

* fix(agents): gate review on all automated checks, not just cargo check

Consolidate check/clippy/fmt/test into a single gate step that
stops and reports if any fail, matching the command spec.

* fix(skills): replace unwrap with expect in channel example

Use .expect("receiver disconnected") on tx.send() for consistency
with the .expect() convention used in all other concurrency examples.

* fix: address final review round — OpenCode mirrors, counts, examples

- Add .opencode/commands/rust-{build,review,test}.md mirrors
- Add .opencode/prompts/agents/rust-{build-resolver,reviewer}.txt mirrors
- Fix AGENTS.md count to 20 (add missing kotlin agents to table)
- Fix review example: all checks pass (consistent with gate policy)
- Replace should_panic doctest with is_err() (consistent with best practices)
- Trim testing commands to stay at 500-line limit

* fix: address cubic and greptile review on OpenCode files and agents

- Fix crate::module import guidance (internal path, not Cargo.toml)
- Add cargo fmt --check to verification steps
- Fix TDD GREEN example to handle error path (validate(input)?)
- Scope .context() guidance to anyhow/eyre application code
- Update command count from 40 to 51
- Add tokio channel variants to unbounded channel warning
- Preserve JoinError context in spawned task panic message

* fix: stale command count, channel guidance, cargo tree fallback

- Fix stale command count in Project Structure section (40→51)
- Clarify unbounded channel rule: context-appropriate bounded alternatives
- Remove dead cargo tree fallback (exits 0 even with no duplicates)
- Sync OpenCode reviewer mirror with tokio channel coverage

.opencode/commands/rust-build.md

@@ -0,0 +1,78 @@
+---
+description: Fix Rust build errors and borrow checker issues
+agent: rust-build-resolver
+subtask: true
+---
+
+# Rust Build Command
+
+Fix Rust build, clippy, and dependency errors: $ARGUMENTS
+
+## Your Task
+
+1. **Run cargo check**: `cargo check 2>&1`
+2. **Run cargo clippy**: `cargo clippy -- -D warnings 2>&1`
+3. **Fix errors** one at a time
+4. **Verify fixes** don't introduce new errors
+
+## Common Rust Errors
+
+### Borrow Checker
+```
+cannot borrow `x` as mutable because it is also borrowed as immutable
+```
+**Fix**: Restructure to end immutable borrow first; clone only if justified
+
+### Type Mismatch
+```
+mismatched types: expected `T`, found `U`
+```
+**Fix**: Add `.into()`, `as`, or explicit type conversion
+
+### Missing Import
+```
+unresolved import `crate::module`
+```
+**Fix**: Fix the `use` path or declare the module (add Cargo.toml deps only for external crates)
+
+### Lifetime Errors
+```
+does not live long enough
+```
+**Fix**: Use owned type or add lifetime annotation
+
+### Trait Not Implemented
+```
+the trait `X` is not implemented for `Y`
+```
+**Fix**: Add `#[derive(Trait)]` or implement manually
+
+## Fix Order
+
+1. **Build errors** - Code must compile
+2. **Clippy warnings** - Fix suspicious constructs
+3. **Formatting** - `cargo fmt` compliance
+
+## Build Commands
+
+```bash
+cargo check 2>&1
+cargo clippy -- -D warnings 2>&1
+cargo fmt --check 2>&1
+cargo tree --duplicates
+cargo test
+```
+
+## Verification
+
+After fixes:
+```bash
+cargo check                  # Should succeed
+cargo clippy -- -D warnings  # No warnings allowed
+cargo fmt --check            # Formatting should pass
+cargo test                   # Tests should pass
+```
+
+---
+
+**IMPORTANT**: Fix errors only. No refactoring, no improvements. Get the build green with minimal changes.

.opencode/commands/rust-review.md

@@ -0,0 +1,65 @@
+---
+description: Rust code review for ownership, safety, and idiomatic patterns
+agent: rust-reviewer
+subtask: true
+---
+
+# Rust Review Command
+
+Review Rust code for idiomatic patterns and best practices: $ARGUMENTS
+
+## Your Task
+
+1. **Analyze Rust code** for idioms and patterns
+2. **Check ownership** - borrowing, lifetimes, unnecessary clones
+3. **Review error handling** - proper `?` propagation, no unwrap in production
+4. **Verify safety** - unsafe usage, injection, secrets
+
+## Review Checklist
+
+### Safety (CRITICAL)
+- [ ] No unchecked `unwrap()`/`expect()` in production paths
+- [ ] `unsafe` blocks have `// SAFETY:` comments
+- [ ] No SQL/command injection
+- [ ] No hardcoded secrets
+
+### Ownership (HIGH)
+- [ ] No unnecessary `.clone()` to satisfy borrow checker
+- [ ] `&str` preferred over `String` in function parameters
+- [ ] `&[T]` preferred over `Vec<T>` in function parameters
+- [ ] No excessive lifetime annotations where elision works
+
+### Error Handling (HIGH)
+- [ ] Errors propagated with `?`; use `.context()` in `anyhow`/`eyre` application code
+- [ ] No silenced errors (`let _ = result;`)
+- [ ] `thiserror` for library errors, `anyhow` for applications
+
+### Concurrency (HIGH)
+- [ ] No blocking in async context
+- [ ] Bounded channels preferred
+- [ ] `Mutex` poisoning handled
+- [ ] `Send`/`Sync` bounds correct
+
+### Code Quality (MEDIUM)
+- [ ] Functions under 50 lines
+- [ ] No deep nesting (>4 levels)
+- [ ] Exhaustive matching on business enums
+- [ ] Clippy warnings addressed
+
+## Report Format
+
+### CRITICAL Issues
+- [file:line] Issue description
+  Suggestion: How to fix
+
+### HIGH Issues
+- [file:line] Issue description
+  Suggestion: How to fix
+
+### MEDIUM Issues
+- [file:line] Issue description
+  Suggestion: How to fix
+
+---
+
+**TIP**: Run `cargo clippy -- -D warnings` and `cargo fmt --check` for automated checks.

.opencode/commands/rust-test.md

@@ -0,0 +1,104 @@
+---
+description: Rust TDD workflow with unit and property tests
+agent: tdd-guide
+subtask: true
+---
+
+# Rust Test Command
+
+Implement using Rust TDD methodology: $ARGUMENTS
+
+## Your Task
+
+Apply test-driven development with Rust idioms:
+
+1. **Define types** - Structs, enums, traits
+2. **Write tests** - Unit tests in `#[cfg(test)]` modules
+3. **Implement minimal code** - Pass the tests
+4. **Check coverage** - Target 80%+
+
+## TDD Cycle for Rust
+
+### Step 1: Define Interface
+```rust
+pub struct Input {
+    // fields
+}
+
+pub fn process(input: &Input) -> Result<Output, Error> {
+    todo!()
+}
+```
+
+### Step 2: Write Tests
+```rust
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn valid_input_succeeds() {
+        let input = Input { /* ... */ };
+        let result = process(&input);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn invalid_input_returns_error() {
+        let input = Input { /* ... */ };
+        let result = process(&input);
+        assert!(result.is_err());
+    }
+}
+```
+
+### Step 3: Run Tests (RED)
+```bash
+cargo test
+```
+
+### Step 4: Implement (GREEN)
+```rust
+pub fn process(input: &Input) -> Result<Output, Error> {
+    // Minimal implementation that handles both paths
+    validate(input)?;
+    Ok(Output { /* ... */ })
+}
+```
+
+### Step 5: Check Coverage
+```bash
+cargo llvm-cov
+cargo llvm-cov --fail-under-lines 80
+```
+
+## Rust Testing Commands
+
+```bash
+cargo test                        # Run all tests
+cargo test -- --nocapture         # Show println output
+cargo test test_name              # Run specific test
+cargo test --no-fail-fast         # Don't stop on first failure
+cargo test --lib                  # Unit tests only
+cargo test --test integration     # Integration tests only
+cargo test --doc                  # Doc tests only
+cargo bench                       # Run benchmarks
+```
+
+## Test File Organization
+
+```
+src/
+├── lib.rs             # Library root
+├── service.rs         # Implementation
+└── service/
+    └── tests.rs       # Or inline #[cfg(test)] mod tests {}
+tests/
+└── integration.rs     # Integration tests
+benches/
+└── benchmark.rs       # Criterion benchmarks
+```
+
+---
+
+**TIP**: Use `rstest` for parameterized tests and `proptest` for property-based testing.