fix: resolve DM channel for agent:main:main sessions

The main agent session uses key 'agent:main:main' which doesn't
contain a channel ID. The session monitor now falls back to reading
deliveryContext/lastTo from sessions.json and resolves 'user:XXXX'
format via the Mattermost direct channel API.

Fixes: status watcher not tracking the main agent's active transcript

src/watcher-manager.js

@@ -206,6 +206,9 @@ async function startDaemon() {
     transcriptDir: config.transcriptDir,
     pollMs: config.sessionPollMs,
     defaultChannel: config.defaultChannel,
+    mmToken: config.mm.token,
+    mmUrl: config.mm.baseUrl,
+    botUserId: config.mm.botUserId,
     logger: logger.child({ module: 'session-monitor' }),
   });