feat(channels): add Mattermost channel support

[evolv3ai]

Summary

• Add Mattermost channel integration using async WebSocket (websockets) + REST API (httpx) - no new dependencies needed
• Thread-aware replies: channel messages auto-thread, DMs stay flat, configurable via reply_in_thread
• Auto-reconnect with exponential backoff (5s-60s) on WebSocket disconnect
• Message splitting at 4000 chars for long responses (Mattermost limit: 16383)
• Bot @mention stripping via configurable username field
• Ported from picoclaw's Go implementation, avoiding the sync/async issues in HKUDS/nanobot#34

Files Changed

• nanobot/config/schema.py - Add MattermostConfig with url, token, username, reply_in_thread, allow_from
• nanobot/channels/mattermost.py - Full channel implementation (~260 lines)
• nanobot/channels/manager.py - Register Mattermost in channel init

Configuration

{
  "channels": {
    "mattermost": {
      "enabled": true,
      "url": "https://your-mattermost-server.com",
      "token": "YOUR_BOT_ACCESS_TOKEN",
      "username": "botname",
      "replyInThread": true,
      "allowFrom": []
    }
  }
}

Test plan

• REST API auth (/api/v4/users/me)
• WebSocket connection + authentication_challenge
• Inbound message receive (DM + channel)
• Outbound message send via channel.send()
• Threaded replies in channels (channelID/rootID encoding)
• Message splitting (11K chars -> 3 chunks, all <= 4000)
• Auto-reconnect after forced WebSocket disconnect (~4s recovery)
• Echo round-trip (receive -> reply)

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