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/config.js

@@ -42,6 +42,7 @@ function buildConfig() {
       token: getEnv('MM_BOT_TOKEN', null, true),
       baseUrl: getEnv('MM_BASE_URL', 'https://slack.solio.tech'),
       maxSockets: getEnvInt('MM_MAX_SOCKETS', 4),
+      botUserId: getEnv('MM_BOT_USER_ID', null),
     },
 
     // Transcript directory (OpenClaw agents)