Add Mattermost Channel

Adds Mattermost DMs, channels, threads, files, reactions, and interactive approval cards. Messages arrive over Mattermost's WebSocket; card clicks return to NanoClaw over an authenticated HTTP callback. Every step is safe to re-run.

Discover the server first

Do this before installing the adapter or asking for a URL. The goal is to reuse a healthy Mattermost the user already has and establish one canonical base URL.