Use @ file references to include skills in commands

Skills have only ~20% auto-activation rate when referenced by name.
Using @~/.claude/skills/*/SKILL.md guarantees skill content is loaded.

Updated all commands to use file references instead of "Use the X skill".
Updated docs/writing-commands.md with new pattern and examples.

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

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

commands/plan-issues.md

@@ -5,7 +5,9 @@ argument-hint: <feature-description>
 
 # Plan Feature: $1
 
-Use the gitea, roadmap-planning, and issue-writing skills.
+@~/.claude/skills/gitea/SKILL.md
+@~/.claude/skills/roadmap-planning/SKILL.md
+@~/.claude/skills/issue-writing/SKILL.md
 
 1. **Understand the feature**: Analyze what "$1" involves
 2. **Explore the codebase** if needed to understand context