feat: add Cursor, Codex, and OpenCode harnesses — maximize every AI coding tool

- AGENTS.md: universal cross-tool file read by Claude Code, Cursor, Codex, and OpenCode
- .cursor/: 15 hook events via hooks.json, 16 hook scripts with DRY adapter pattern,
  29 rules (9 common + 20 language-specific) with Cursor YAML frontmatter
- .codex/: reference config.toml, Codex-specific AGENTS.md supplement,
  10 skills ported to .agents/skills/ with openai.yaml metadata
- .opencode/: 3 new tools (format-code, lint-check, git-summary), 3 new hooks
  (shell.env, experimental.session.compacting, permission.ask), expanded instructions,
  version bumped to 1.6.0
- README: fixed Cursor section, added Codex section, added cross-tool parity table
- install.sh: now copies hooks.json + hooks/ for --target cursor

.cursor/rules/common-coding-style.md

@@ -0,0 +1,52 @@
+---
+description: "ECC coding style: immutability, file organization, error handling, validation"
+alwaysApply: true
+---
+# Coding Style
+
+## Immutability (CRITICAL)
+
+ALWAYS create new objects, NEVER mutate existing ones:
+
+```
+// Pseudocode
+WRONG:  modify(original, field, value) → changes original in-place
+CORRECT: update(original, field, value) → returns new copy with change
+```
+
+Rationale: Immutable data prevents hidden side effects, makes debugging easier, and enables safe concurrency.
+
+## File Organization
+
+MANY SMALL FILES > FEW LARGE FILES:
+- High cohesion, low coupling
+- 200-400 lines typical, 800 max
+- Extract utilities from large modules
+- Organize by feature/domain, not by type
+
+## Error Handling
+
+ALWAYS handle errors comprehensively:
+- Handle errors explicitly at every level
+- Provide user-friendly error messages in UI-facing code
+- Log detailed error context on the server side
+- Never silently swallow errors
+
+## Input Validation
+
+ALWAYS validate at system boundaries:
+- Validate all user input before processing
+- Use schema-based validation where available
+- Fail fast with clear error messages
+- Never trust external data (API responses, user input, file content)
+
+## Code Quality Checklist
+
+Before marking work complete:
+- [ ] Code is readable and well-named
+- [ ] Functions are small (<50 lines)
+- [ ] Files are focused (<800 lines)
+- [ ] No deep nesting (>4 levels)
+- [ ] Proper error handling
+- [ ] No hardcoded values (use constants or config)
+- [ ] No mutation (immutable patterns used)