Mark skills as not user-invocable

Skills are knowledge modules referenced by commands, not
directly invoked by users. Added user-invocable: false to:
- backlog-grooming (used by /groom)
- claude-md-writing (used by /update-claude-md)
- code-review (used by /review-pr)
- issue-writing (used by /create-issue)
- roadmap-planning (used by /plan-issues)
- vision-management (used by /vision, /manifesto)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/backlog-grooming/SKILL.md

@@ -1,6 +1,7 @@
 ---
 name: backlog-grooming
 description: Review and improve existing issues for clarity and actionability. Use when grooming the backlog, reviewing issue quality, cleaning up stale issues, or when the user wants to improve existing issues.
+user-invocable: false
 ---
 
 # Backlog Grooming