What does this PR do?

Adds @-mention-only filtering to the Mattermost gateway adapter. The bot now only responds to messages in channels and groups when it is explicitly @-mentioned. DMs are always processed without filtering.

Currently, the Mattermost adapter processes every message from other users in channels, creating noise and unnecessary API calls. This brings Mattermost in line with Discord and Slack adapters which already filter for mentions.

Related Issue

Fixes #2174

Type of Change

• ✨ New feature (non-breaking change that adds functionality)

Changes Made

• gateway/platforms/mattermost.py: Added mention detection in _handle_post() after the existing "For channels, check for @mention" comment (line 580). Non-DM messages without a matching @username or @user_id in the message text are skipped with a debug log.

How to Test

1. Connect the bot to a Mattermost server with MATTERMOST_ALLOW_ALL_USERS=true
2. Send a message in a channel without @-mentioning the bot — it should not respond
3. Send a message in a channel with @hermes-agent — it should respond normally
4. Send a DM to the bot — it should respond without requiring a mention
5. Check logs for Mattermost: skipping non-DM message without @mention debug entries

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (feat(gateway): ...)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've tested on my platform: Docker (python:3.12-slim on macOS/OrbStack)

Documentation & Housekeeping

• I've considered cross-platform impact (Windows, macOS) — or N/A (WebSocket filtering is platform-independent)
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A