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/golang-testing.md

@@ -0,0 +1,30 @@
+---
+description: "Go testing extending common rules"
+globs: ["**/*.go", "**/go.mod", "**/go.sum"]
+alwaysApply: false
+---
+# Go Testing
+
+> This file extends the common testing rule with Go specific content.
+
+## Framework
+
+Use the standard `go test` with **table-driven tests**.
+
+## Race Detection
+
+Always run with the `-race` flag:
+
+```bash
+go test -race ./...
+```
+
+## Coverage
+
+```bash
+go test -cover ./...
+```
+
+## Reference
+
+See skill: `golang-testing` for detailed Go testing patterns and helpers.