feat(imessage): port permission-relay + lifecycle fixes from telegram

Brings the imessage channel to parity with recent telegram/discord
hardening:

- Permission-relay capability: declare claude/channel/permission,
  handle inbound permission_request notifications by fanning out to
  allowlisted DM chats + self-chat, intercept "yes/no <id>" replies
  after the gate check and emit structured permission events instead
  of relaying as chat. Groups excluded per single-user-mode policy.
- Global unhandledRejection/uncaughtException handlers so the server
  logs instead of dying silently.
- IMESSAGE_STATE_DIR env override for the state directory.
- .unref() on both setInterval timers so they don't block shutdown.
- stdin EOF / SIGTERM / SIGINT shutdown handler that closes chat.db
  and exits cleanly instead of leaving a zombie poll loop.

Adds zod as a direct dep (already transitively present via the MCP SDK)
for the notification handler schema.

external_plugins/imessage/package.json

@@ -8,7 +8,8 @@
     "start": "bun install --no-summary && bun server.ts"
   },
   "dependencies": {
-    "@modelcontextprotocol/sdk": "^1.0.0"
+    "@modelcontextprotocol/sdk": "^1.0.0",
+    "zod": "^3.23.8"
   },
   "devDependencies": {
     "@types/bun": "^1.3.10"