feat: template-ify all skills + E2E tests for plan-ceo-review, plan-eng-review, retro

- Convert gstack-upgrade to SKILL.md.tmpl template system
- All 10 skills now use templates (consistent auto-generated headers)
- Add comprehensive template validation tests (22 tests):
  every skill has .tmpl, generated SKILL.md has header, valid frontmatter,
  --dry-run reports FRESH, no unresolved placeholders
- Add E2E tests for /plan-ceo-review, /plan-eng-review, /retro
- Mark /ship, /setup-browser-cookies, /gstack-upgrade as test.todo (destructive/interactive)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gstack-upgrade/SKILL.md

@@ -9,6 +9,8 @@ allowed-tools:
   - Read
   - AskUserQuestion
 ---
+<!-- AUTO-GENERATED from SKILL.md.tmpl — do not edit directly -->
+<!-- Regenerate: bun run gen:skill-docs -->
 
 # /gstack-upgrade