Add skills frontmatter to issue-worker agent

Background agents need skills specified in frontmatter rather than
using @ syntax which may not expand for Task-spawned agents.

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

agents/issue-worker/agent.md

@@ -2,6 +2,7 @@
 name: issue-worker
 description: Autonomous agent that works on a single issue in an isolated git worktree
 tools: Bash, Read, Write, Edit, Glob, Grep, TodoWrite
+skills: gitea, issue-writing
 ---
 
 # Issue Worker Agent