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-unfreeze/SKILL.md

@@ -0,0 +1,36 @@
+---
+name: unfreeze
+description: |
+  Clear the freeze boundary set by /freeze, allowing edits to all directories
+  again. Use when you want to widen edit scope without ending the session.
+  Use when asked to "unfreeze", "unlock edits", "remove freeze", or
+  "allow all edits".
+---
+<!-- AUTO-GENERATED from SKILL.md.tmpl — do not edit directly -->
+<!-- Regenerate: bun run gen:skill-docs -->
+
+# /unfreeze — Clear Freeze Boundary
+
+Remove the edit restriction set by `/freeze`, allowing edits to all directories.
+
+```bash
+mkdir -p ~/.gstack/analytics
+echo '{"skill":"unfreeze","ts":"'$(date -u +%Y-%m-%dT%H:%M:%SZ)'","repo":"'$(basename "$(git rev-parse --show-toplevel 2>/dev/null)" 2>/dev/null || echo "unknown")'"}'  >> ~/.gstack/analytics/skill-usage.jsonl 2>/dev/null || true
+```
+
+## Clear the boundary
+
+```bash
+STATE_DIR="${CLAUDE_PLUGIN_DATA:-$HOME/.gstack}"
+if [ -f "$STATE_DIR/freeze-dir.txt" ]; then
+  PREV=$(cat "$STATE_DIR/freeze-dir.txt")
+  rm -f "$STATE_DIR/freeze-dir.txt"
+  echo "Freeze boundary cleared (was: $PREV). Edits are now allowed everywhere."
+else
+  echo "No freeze boundary was set."
+fi
+```
+
+Tell the user the result. Note that `/freeze` hooks are still registered for the
+session — they will just allow everything since no state file exists. To re-freeze,
+run `/freeze` again.