Add YAML frontmatter to all skills for automatic discovery

Skills require YAML frontmatter with name and description fields
for Claude Code to automatically discover and load them. Added
frontmatter to all five skill files:
- gitea: CLI for issues, PRs, and repository management
- code-review: Guidelines for reviewing code changes
- issue-writing: How to write clear, actionable issues
- backlog-grooming: Review and improve existing issues
- roadmap-planning: Plan features and create issues

Closes #12

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

skills/code-review/SKILL.md

@@ -1,3 +1,8 @@
+---
+name: code-review
+description: Guidelines and templates for reviewing code changes in pull requests
+---
+
 # Code Review
 
 Guidelines for reviewing code changes in pull requests.