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/work-issue.md

@@ -5,7 +5,7 @@ argument-hint: <issue-number>
 
 # Work on Issue #$1
 
-Use the gitea skill.
+@~/.claude/skills/gitea/SKILL.md
 
 1. **View the issue** to understand requirements
 2. **Create a branch**: `git checkout -b issue-$1-<short-kebab-title>`