Apply fixes from closed PRs: sentinel markers, JID lookup, schedule validation

- PR #10: Add sentinel markers for robust JSON parsing between container
  and host. Fallback to last-line parsing for backwards compatibility.

- PR #5: Look up target JID from registeredGroups instead of trusting
  IPC payload, fixing cross-group scheduled tasks getting wrong chat_jid.

- PR #8: Add lightweight schedule validation in container MCP that
  returns errors to agents (cron syntax, positive interval, valid ISO
  timestamp). Also defensive validation on host side.

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

container/agent-runner/package.json

@@ -10,6 +10,7 @@
   },
   "dependencies": {
     "@anthropic-ai/claude-agent-sdk": "0.2.29",
+    "cron-parser": "^5.0.0",
     "zod": "^4.0.0"
   },
   "devDependencies": {