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>

src/container-runner.ts

@@ -21,6 +21,10 @@ const logger = pino({
   transport: { target: 'pino-pretty', options: { colorize: true } }
 });
 
+// Sentinel markers for robust output parsing (must match agent-runner)
+const OUTPUT_START_MARKER = '---NANOCLAW_OUTPUT_START---';
+const OUTPUT_END_MARKER = '---NANOCLAW_OUTPUT_END---';
+
 function getHomeDir(): string {
   const home = process.env.HOME || os.homedir();
   if (!home) {
@@ -321,9 +325,19 @@ export async function runContainerAgent(
       }
 
       try {
-        // Last non-empty line is the JSON output
-        const lines = stdout.trim().split('\n');
-        const jsonLine = lines[lines.length - 1];
+        // Extract JSON between sentinel markers for robust parsing
+        const startIdx = stdout.indexOf(OUTPUT_START_MARKER);
+        const endIdx = stdout.indexOf(OUTPUT_END_MARKER);
+
+        let jsonLine: string;
+        if (startIdx !== -1 && endIdx !== -1 && endIdx > startIdx) {
+          jsonLine = stdout.slice(startIdx + OUTPUT_START_MARKER.length, endIdx).trim();
+        } else {
+          // Fallback: last non-empty line (backwards compatibility)
+          const lines = stdout.trim().split('\n');
+          jsonLine = lines[lines.length - 1];
+        }
+
         const output: ContainerOutput = JSON.parse(jsonLine);
 
         logger.info({