fix: skip heartbeat/internal sessions (agent:main:main) from status box creation

Heartbeat sessions (key pattern agent:<agent>:main) have no real Mattermost
conversation context. The daemon was resolving them to the DM fallback channel
and creating a new status box on every heartbeat cycle (~every 30min but firing
rapidly during active work). Each one appeared as a separate live status post.

Fix: in session-added handler, skip any session key matching /^agent:[^:]+:main$/
or /^agent:[^:]+:cli$/ before creating a status box.

src/watcher-manager.js

@@ -262,6 +262,14 @@ async function startDaemon() {
       return;
     }
 
+    // Skip heartbeat/internal sessions (agent:main:main, agent:main:cli, etc.)
+    // These have no real Mattermost conversation context and produce spurious status boxes.
+    // A heartbeat session key ends with ':main' and has no channel/thread suffix.
+    if (/^agent:[^:]+:main$/.test(sessionKey) || /^agent:[^:]+:cli$/.test(sessionKey)) {
+      logger.debug({ sessionKey }, 'Heartbeat/internal session — skipping status box');
+      return;
+    }
+
     // Enforce MAX_ACTIVE_SESSIONS
     if (activeBoxes.size >= config.maxActiveSessions) {
       logger.warn(