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

commands/rust-build.md

@@ -0,0 +1,187 @@
+---
+description: Fix Rust build errors, borrow checker issues, and dependency problems incrementally. Invokes the rust-build-resolver agent for minimal, surgical fixes.
+---
+
+# Rust Build and Fix
+
+This command invokes the **rust-build-resolver** agent to incrementally fix Rust build errors with minimal changes.
+
+## What This Command Does
+
+1. **Run Diagnostics**: Execute `cargo check`, `cargo clippy`, `cargo fmt --check`
+2. **Parse Errors**: Identify error codes and affected files
+3. **Fix Incrementally**: One error at a time
+4. **Verify Each Fix**: Re-run `cargo check` after each change
+5. **Report Summary**: Show what was fixed and what remains
+
+## When to Use
+
+Use `/rust-build` when:
+- `cargo build` or `cargo check` fails with errors
+- `cargo clippy` reports warnings
+- Borrow checker or lifetime errors block compilation
+- Cargo dependency resolution fails
+- After pulling changes that break the build
+
+## Diagnostic Commands Run
+
+```bash
+# Primary build check
+cargo check 2>&1
+
+# Lints and suggestions
+cargo clippy -- -D warnings 2>&1
+
+# Formatting check
+cargo fmt --check 2>&1
+
+# Dependency issues
+cargo tree --duplicates
+
+# Security audit (if available)
+if command -v cargo-audit >/dev/null; then cargo audit; else echo "cargo-audit not installed"; fi
+```
+
+## Example Session
+
+````text
+User: /rust-build
+
+Agent:
+# Rust Build Resolution
+
+## Initial Diagnostics
+
+```bash
+$ cargo check
+error[E0502]: cannot borrow `map` as mutable because it is also borrowed as immutable
+  --> src/service/cache.rs:25:5
+error[E0308]: mismatched types
+  --> src/handler/api.rs:42:12
+error[E0425]: cannot find value `db` in this scope
+  --> src/handler/api.rs:58:9
+```
+
+Errors found: 3
+
+## Fix 1: Borrow Checker Error
+
+File: src/service/cache.rs:25
+Error: E0502 — cannot borrow `map` as mutable because also borrowed as immutable
+Cause: Immutable borrow still active during mutable insert
+
+```rust
+// Changed: restructured to end immutable borrow before mutable access
+if !map.contains_key("key") {
+    map.insert("key".into(), default);
+}
+```
+
+```bash
+$ cargo check
+# 2 errors remaining
+```
+
+## Fix 2: Type Mismatch
+
+File: src/handler/api.rs:42
+Error: E0308 — mismatched types: expected `usize`, found `&str`
+
+```rust
+// Changed
+let count = params.get("count");
+// To
+let count: usize = params.get("count")
+    .and_then(|s| s.parse().ok())
+    .unwrap_or(0);
+```
+
+```bash
+$ cargo check
+# 1 error remaining
+```
+
+## Fix 3: Unresolved Name
+
+File: src/handler/api.rs:58
+Error: E0425 — cannot find value `db` in this scope
+Cause: Missing import
+
+```rust
+// Added
+use crate::db;
+```
+
+```bash
+$ cargo check
+# Build successful!
+```
+
+## Final Verification
+
+```bash
+$ cargo clippy -- -D warnings
+# No warnings
+
+$ cargo test
+running 6 tests
+test service::cache::tests::test_insert ... ok
+test service::cache::tests::test_get ... ok
+test handler::api::tests::test_parse_count ... ok
+test handler::api::tests::test_missing_count ... ok
+test handler::api::tests::test_db_import ... ok
+test handler::api::tests::test_response ... ok
+
+test result: ok. 6 passed; 0 failed; 0 ignored
+```
+
+## Summary
+
+| Metric | Count |
+|--------|-------|
+| Build errors fixed | 3 |
+| Clippy warnings fixed | 0 |
+| Files modified | 2 |
+| Remaining issues | 0 |
+
+Build Status: SUCCESS
+````
+
+## Common Errors Fixed
+
+| Error | Typical Fix |
+|-------|-------------|
+| `cannot borrow as mutable` | Restructure to end immutable borrow first; clone only if justified |
+| `does not live long enough` | Use owned type or add lifetime annotation |
+| `cannot move out of` | Restructure to take ownership; clone only as last resort |
+| `mismatched types` | Add `.into()`, `as`, or explicit conversion |
+| `trait X not implemented` | Add `#[derive(Trait)]` or implement manually |
+| `unresolved import` | Add to Cargo.toml or fix `use` path |
+| `cannot find value` | Add import or fix path |
+
+## Fix Strategy
+
+1. **Build errors first** - Code must compile
+2. **Clippy warnings second** - Fix suspicious constructs
+3. **Formatting third** - `cargo fmt` compliance
+4. **One fix at a time** - Verify each change
+5. **Minimal changes** - Don't refactor, just fix
+
+## Stop Conditions
+
+The agent will stop and report if:
+- Same error persists after 3 attempts
+- Fix introduces more errors
+- Requires architectural changes
+- Borrow checker error requires redesigning data ownership
+
+## Related Commands
+
+- `/rust-test` - Run tests after build succeeds
+- `/rust-review` - Review code quality
+- `/verify` - Full verification loop
+
+## Related
+
+- Agent: `agents/rust-build-resolver.md`
+- Skill: `skills/rust-patterns/`