Improve skill descriptions and documentation

Updated all skill descriptions with proper trigger terms following
the pattern: "What it does. Use when [trigger terms]."

Skills updated:
- code-review: triggers on PR review, code quality, bugs, security
- issue-writing: triggers on creating issues, bug reports, features
- backlog-grooming: triggers on grooming, reviewing issue quality
- roadmap-planning: triggers on planning features, breaking down work

Updated docs/writing-skills.md:
- Added YAML frontmatter requirements section
- Documented required and optional fields
- Added guidance on writing effective descriptions
- Updated "How Skills are Loaded" to reflect model-invoked behavior
- Added note about subagent skill access
- Updated checklist with frontmatter requirements
- Added reference to official documentation

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

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

skills/roadmap-planning/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: roadmap-planning
-description: How to plan features and create issues for implementation
+description: Plan features and break down work into implementable issues. Use when planning a feature, creating a roadmap, breaking down large tasks, or when the user needs help organizing work into issues.
 ---
 
 # Roadmap Planning