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/src/index.ts

@@ -45,8 +45,13 @@ async function readStdin(): Promise<string> {
   });
 }
 
+const OUTPUT_START_MARKER = '---NANOCLAW_OUTPUT_START---';
+const OUTPUT_END_MARKER = '---NANOCLAW_OUTPUT_END---';
+
 function writeOutput(output: ContainerOutput): void {
+  console.log(OUTPUT_START_MARKER);
   console.log(JSON.stringify(output));
+  console.log(OUTPUT_END_MARKER);
 }
 
 function log(message: string): void {