feat: generate Codex/Gemini/Cursor skills into .agents/skills/

Generated 21 skill files for the open SKILL.md standard:
- Output: .agents/skills/gstack-*/SKILL.md (one per skill)
- Frontmatter: name + description only (no allowed-tools/version)
- No .claude/skills/ paths in any generated file
- /codex skill excluded (Claude wrapper, self-referential on Codex)
- Hook skills (careful/freeze/guard) get inline safety prose
- Build script generates both hosts: bun run build

Supported agents (all read .agents/skills/):
- Codex CLI
- Gemini CLI
- Cursor

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.agents/skills/gstack-careful/SKILL.md

@@ -0,0 +1,50 @@
+---
+name: careful
+description: |
+  Safety guardrails for destructive commands. Warns before rm -rf, DROP TABLE,
+  force-push, git reset --hard, kubectl delete, and similar destructive operations.
+  User can override each warning. Use when touching prod, debugging live systems,
+  or working in a shared environment. Use when asked to "be careful", "safety mode",
+  "prod mode", or "careful mode".
+---
+<!-- AUTO-GENERATED from SKILL.md.tmpl — do not edit directly -->
+<!-- Regenerate: bun run gen:skill-docs -->
+> **Safety Advisory:** This skill includes safety checks that check bash commands for destructive operations (rm -rf, DROP TABLE, force-push, git reset --hard, etc.) before execution. When using this skill, always pause and verify before executing potentially destructive operations. If uncertain about a command's safety, ask the user for confirmation before proceeding.
+
+
+# /careful — Destructive Command Guardrails
+
+Safety mode is now **active**. Every bash command will be checked for destructive
+patterns before running. If a destructive command is detected, you'll be warned
+and can choose to proceed or cancel.
+
+```bash
+mkdir -p ~/.gstack/analytics
+echo '{"skill":"careful","ts":"'$(date -u +%Y-%m-%dT%H:%M:%SZ)'","repo":"'$(basename "$(git rev-parse --show-toplevel 2>/dev/null)" 2>/dev/null || echo "unknown")'"}'  >> ~/.gstack/analytics/skill-usage.jsonl 2>/dev/null || true
+```
+
+## What's protected
+
+| Pattern | Example | Risk |
+|---------|---------|------|
+| `rm -rf` / `rm -r` / `rm --recursive` | `rm -rf /var/data` | Recursive delete |
+| `DROP TABLE` / `DROP DATABASE` | `DROP TABLE users;` | Data loss |
+| `TRUNCATE` | `TRUNCATE orders;` | Data loss |
+| `git push --force` / `-f` | `git push -f origin main` | History rewrite |
+| `git reset --hard` | `git reset --hard HEAD~3` | Uncommitted work loss |
+| `git checkout .` / `git restore .` | `git checkout .` | Uncommitted work loss |
+| `kubectl delete` | `kubectl delete pod` | Production impact |
+| `docker rm -f` / `docker system prune` | `docker system prune -a` | Container/image loss |
+
+## Safe exceptions
+
+These patterns are allowed without warning:
+- `rm -rf node_modules` / `.next` / `dist` / `__pycache__` / `.cache` / `build` / `.turbo` / `coverage`
+
+## How it works
+
+The hook reads the command from the tool input JSON, checks it against the
+patterns above, and returns `permissionDecision: "ask"` with a warning message
+if a match is found. You can always override the warning and proceed.
+
+To deactivate, end the conversation or start a new one. Hooks are session-scoped.