fix: dedup status boxes — one per channel, thread sessions take priority

When a bare channel session (no 🧵 suffix) and a thread-specific session
both resolve to the same MM channel/DM, two status boxes appeared simultaneously.

Fix: in session-added handler, before creating a box, check if any existing
active session already owns that channelId. Thread sessions displace bare channel
sessions (and take priority). Bare channel sessions are skipped if a thread
session already exists. First-in wins for same-type duplicates.

src/watcher-manager.js

@@ -270,6 +270,32 @@ async function startDaemon() {
       return;
     }
 
+    // Dedup: skip if another active session already owns this channel.
+    // This prevents duplicate status boxes when a threadless parent session and a
+    // thread-specific child session both resolve to the same MM channel/DM.
+    // Thread sessions (containing ':thread:') take priority over bare channel sessions.
+    const isThreadSession = sessionKey.includes(':thread:');
+    for (const [existingKey, existingBox] of activeBoxes) {
+      if (existingBox.channelId === channelId) {
+        const existingIsThread = existingKey.includes(':thread:');
+        if (isThreadSession && !existingIsThread) {
+          // New session is a thread — it takes priority. Remove the parent box.
+          logger.info({ sessionKey, displaced: existingKey }, 'Thread session displacing bare channel session');
+          activeBoxes.delete(existingKey);
+          watcher.removeSession(existingKey);
+        } else if (!isThreadSession && existingIsThread) {
+          // Existing session is a thread — skip this bare channel session.
+          logger.debug({ sessionKey, existingKey }, 'Bare channel session skipped — thread session already owns this channel');
+          return;
+        } else {
+          // Same type — skip the newcomer, first-in wins.
+          logger.debug({ sessionKey, existingKey }, 'Duplicate channel session skipped');
+          return;
+        }
+        break;
+      }
+    }
+
     // Enforce MAX_ACTIVE_SESSIONS
     if (activeBoxes.size >= config.maxActiveSessions) {
       logger.warn(