feat: add setup skill with scripted steps (#258)

Replace inline SKILL.md instructions with executable shell scripts
for each setup phase (environment check, deps, container, auth,
groups, channels, mounts, service, verify). Scripts emit structured
status blocks for reliable parsing.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/router.ts

@@ -1,6 +1,7 @@
 import { Channel, NewMessage } from './types.js';
 
 export function escapeXml(s: string): string {
+  if (!s) return '';
   return s
     .replace(/&/g, '&')
     .replace(/</g, '&lt;')