fix: re-detect sessions after idle cleanup

Added forgetSession() to SessionMonitor. When watcher marks a session
idle/done, it now clears the key from the monitor's known sessions map.
Next poll cycle re-detects the session if the transcript is still active,
creating a fresh status post.

src/watcher-manager.js

@@ -339,9 +339,10 @@ async function startDaemon() {
       logger.warn({ sessionKey, err: unpinErr }, 'Failed to unpin status post');
     }
 
-    // Clean up
+    // Clean up — remove from all tracking so session can be re-detected if it becomes active again
     activeBoxes.delete(sessionKey);
     watcher.removeSession(sessionKey);
+    monitor.forgetSession(sessionKey);
     globalMetrics.activeSessions = activeBoxes.size;
 
     // Persist final offsets