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-testing.md

@@ -0,0 +1,33 @@
+---
+description: "Testing requirements: 80% coverage, TDD workflow, test types"
+alwaysApply: true
+---
+# Testing Requirements
+
+## Minimum Test Coverage: 80%
+
+Test Types (ALL required):
+1. **Unit Tests** - Individual functions, utilities, components
+2. **Integration Tests** - API endpoints, database operations
+3. **E2E Tests** - Critical user flows (framework chosen per language)
+
+## Test-Driven Development
+
+MANDATORY workflow:
+1. Write test first (RED)
+2. Run test - it should FAIL
+3. Write minimal implementation (GREEN)
+4. Run test - it should PASS
+5. Refactor (IMPROVE)
+6. Verify coverage (80%+)
+
+## Troubleshooting Test Failures
+
+1. Use **tdd-guide** agent
+2. Check test isolation
+3. Verify mocks are correct
+4. Fix implementation, not tests (unless tests are wrong)
+
+## Agent Support
+
+- **tdd-guide** - Use PROACTIVELY for new features, enforces write-tests-first