add-mattermost

# 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. 1. Check an existing `MATTERMOST_BASE_URL` in the current environment and NanoClaw env/config files. Do not print tokens or dump whole env files. 2. Probe likely local URLs, at least `http://localhost:8065` and `http://127.0.0.1:8065`, using `GET /api/v4/system/ping`. A listening port alone is not evidence that the service is Mattermost. 3. Inspect Docker/Compose for Mattermost containers. If a matching container exists but is stopped, offer to start it with its original mechanism; do not start or recreate it without the user's approval. 4. If you find a healthy server, show its URL and ask the user to use it or enter a different URL. Do not select a server automatically. Treat the localhost and 127.0.0.1 endpoints for the same container as one server. 5. If nothing healthy is found, offer Mattermost's maintained evaluation and deployment guidance from [SERVER_SETUP.md](SERVER_SETUP.md), then ask for the server URL after the operator has one running. Set `MATTERMOST_BASE_URL` to the chosen canonical URL (s...