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/code-review/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: code-review
-description: Guidelines and templates for reviewing code changes in pull requests
+description: Review code for quality, bugs, security, and style issues. Use when reviewing pull requests, checking code quality, looking for bugs or security vulnerabilities, or when the user asks for a code review.
 ---
 
 # Code Review