fix: concurrent session-added dedup via sessionAddInProgress set

Root cause of double status boxes: lock file event + ghost watch both fire
at the same time on reactivation. Both call clearCompleted+pollNow, both
session-added events reach the handler before activeBoxes.has() returns true
for either, so two status boxes are created.

Fix: sessionAddInProgress Set gates the handler. First caller proceeds,
second caller sees the key in-progress and returns immediately. Cleared
on success (after activeBoxes.set) and on error (before return).

src/watcher-manager.js

@@ -160,6 +160,8 @@ async function startDaemon() {
   // Shared state
   // Map<sessionKey, { postId, agentId, channelId, rootPostId, children: Map }>
   const activeBoxes = new Map();
+  // Guard against concurrent session-added events for the same key (e.g. lock + ghost fire simultaneously)
+  const sessionAddInProgress = new Set();
 
   // Completed sessions: Map<sessionKey, { postId, lastOffset }>
   // Tracks sessions that went idle so we can reuse their post on reactivation
@@ -256,6 +258,19 @@ async function startDaemon() {
   monitor.on('session-added', async (info) => {
     const { sessionKey, transcriptFile, spawnedBy, channelId, rootPostId, agentId } = info;
 
+    // Guard: prevent duplicate concurrent session-added for same key.
+    // Happens when lock file event + ghost watch both fire simultaneously,
+    // both call pollNow(), and both session-added events land before activeBoxes is updated.
+    if (sessionAddInProgress.has(sessionKey)) {
+      logger.debug({ sessionKey }, 'session-added already in progress — dedup skip');
+      return;
+    }
+    if (activeBoxes.has(sessionKey)) {
+      logger.debug({ sessionKey }, 'session-added for already-active session — skip');
+      return;
+    }
+    sessionAddInProgress.add(sessionKey);
+
     // Skip if no channel
     if (!channelId) {
       logger.debug({ sessionKey }, 'No channel for session — skipping');
@@ -374,6 +389,7 @@ async function startDaemon() {
       } catch (err) {
         logger.error({ sessionKey, err }, 'Failed to create status post');
         globalMetrics.lastError = err.message;
+        sessionAddInProgress.delete(sessionKey);
         return;
       }
     }
@@ -386,6 +402,7 @@ async function startDaemon() {
       usePlugin: usePlugin && !!pluginClient, // track which mode this session uses
       children: new Map(),
     });
+    sessionAddInProgress.delete(sessionKey);
     globalMetrics.activeSessions = activeBoxes.size;
 
     // Register in watcher.