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>

.claude/settings.json

@@ -1,10 +1,5 @@
 {
   "sandbox": {
-    "network": {
-      "allowedDomains": [
-        "npm.registry.com",
-        "us.i.posthog.com"
-      ]
-    }
+    "enabled": false
   }
 }