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-guard/SKILL.md

@@ -0,0 +1,62 @@
+---
+name: guard
+description: |
+  Full safety mode: destructive command warnings + directory-scoped edits.
+  Combines /careful (warns before rm -rf, DROP TABLE, force-push, etc.) with
+  /freeze (blocks edits outside a specified directory). Use for maximum safety
+  when touching prod or debugging live systems. Use when asked to "guard mode",
+  "full safety", "lock it down", or "maximum safety".
+---
+<!-- 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, and verify file edits are within the allowed scope boundary before applying, and verify file writes are within the allowed scope boundary before applying. 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.
+
+
+# /guard — Full Safety Mode
+
+Activates both destructive command warnings and directory-scoped edit restrictions.
+This is the combination of `/careful` + `/freeze` in a single command.
+
+**Dependency note:** This skill references hook scripts from the sibling `/careful`
+and `/freeze` skill directories. Both must be installed (they are installed together
+by the gstack setup script).
+
+```bash
+mkdir -p ~/.gstack/analytics
+echo '{"skill":"guard","ts":"'$(date -u +%Y-%m-%dT%H:%M:%SZ)'","repo":"'$(basename "$(git rev-parse --show-toplevel 2>/dev/null)" 2>/dev/null || echo "unknown")'"}'  >> ~/.gstack/analytics/skill-usage.jsonl 2>/dev/null || true
+```
+
+## Setup
+
+Ask the user which directory to restrict edits to. Use AskUserQuestion:
+
+- Question: "Guard mode: which directory should edits be restricted to? Destructive command warnings are always on. Files outside the chosen path will be blocked from editing."
+- Text input (not multiple choice) — the user types a path.
+
+Once the user provides a directory path:
+
+1. Resolve it to an absolute path:
+```bash
+FREEZE_DIR=$(cd "<user-provided-path>" 2>/dev/null && pwd)
+echo "$FREEZE_DIR"
+```
+
+2. Ensure trailing slash and save to the freeze state file:
+```bash
+FREEZE_DIR="${FREEZE_DIR%/}/"
+STATE_DIR="${CLAUDE_PLUGIN_DATA:-$HOME/.gstack}"
+mkdir -p "$STATE_DIR"
+echo "$FREEZE_DIR" > "$STATE_DIR/freeze-dir.txt"
+echo "Freeze boundary set: $FREEZE_DIR"
+```
+
+Tell the user:
+- "**Guard mode active.** Two protections are now running:"
+- "1. **Destructive command warnings** — rm -rf, DROP TABLE, force-push, etc. will warn before executing (you can override)"
+- "2. **Edit boundary** — file edits restricted to `<path>/`. Edits outside this directory are blocked."
+- "To remove the edit boundary, run `/unfreeze`. To deactivate everything, end the session."
+
+## What's protected
+
+See `/careful` for the full list of destructive command patterns and safe exceptions.
+See `/freeze` for how edit boundary enforcement works.