improve setup flow: streamline steps, add pre-approval, new manage-mounts skill

- Disable sandbox by default in project settings
- Setup: remove Apple Container option (Docker only), single channel selection
  with plain text list, move fork to end, auto-set empty mounts, add command
  pre-approval step, add UTC timezone confirmation, add wait-on-user guidance,
  add 5m timeouts for long steps
- iMessage: improve Full Disk Access UX with Finder open + drag instructions
- Add /manage-mounts skill for post-setup mount configuration
- Enable iMessage channel import

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/channels/index.ts

@@ -35,7 +35,7 @@ import './telegram.js';
 // import './webex.js';
 
 // imessage
-// import './imessage.js';
+import './imessage.js';
 
 // gmail (native, no Chat SDK)