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

.codex/AGENTS.md

@@ -0,0 +1,55 @@
+# ECC for Codex CLI
+
+This supplements the root `AGENTS.md` with Codex-specific guidance.
+
+## Model Recommendations
+
+| Task Type | Recommended Model |
+|-----------|------------------|
+| Routine coding, tests, formatting | o4-mini |
+| Complex features, architecture | o3 |
+| Debugging, refactoring | o4-mini |
+| Security review | o3 |
+
+## Skills Discovery
+
+Skills are auto-loaded from `.agents/skills/`. Each skill contains:
+- `SKILL.md` — Detailed instructions and workflow
+- `agents/openai.yaml` — Codex interface metadata
+
+Available skills:
+- tdd-workflow — Test-driven development with 80%+ coverage
+- security-review — Comprehensive security checklist
+- coding-standards — Universal coding standards
+- frontend-patterns — React/Next.js patterns
+- backend-patterns — API design, database, caching
+- e2e-testing — Playwright E2E tests
+- eval-harness — Eval-driven development
+- strategic-compact — Context management
+- api-design — REST API design patterns
+- verification-loop — Build, test, lint, typecheck, security
+
+## MCP Servers
+
+Configure in `~/.codex/config.toml` under `[mcp_servers]`. See `.codex/config.toml` for reference configuration with GitHub, Context7, Memory, and Sequential Thinking servers.
+
+## Key Differences from Claude Code
+
+| Feature | Claude Code | Codex CLI |
+|---------|------------|-----------|
+| Hooks | 8+ event types | Not yet supported |
+| Context file | CLAUDE.md + AGENTS.md | AGENTS.md only |
+| Skills | Skills loaded via plugin | `.agents/skills/` directory |
+| Commands | `/slash` commands | Instruction-based |
+| Agents | Subagent Task tool | Single agent model |
+| Security | Hook-based enforcement | Instruction + sandbox |
+| MCP | Full support | Command-based only |
+
+## Security Without Hooks
+
+Since Codex lacks hooks, security enforcement is instruction-based:
+1. Always validate inputs at system boundaries
+2. Never hardcode secrets — use environment variables
+3. Run `npm audit` / `pip audit` before committing
+4. Review `git diff` before every push
+5. Use `sandbox_mode = "workspace-write"` in config