feat(mattermost): configurable mention behavior — respond without @mention (#3664)

Adds MATTERMOST_REQUIRE_MENTION and MATTERMOST_FREE_RESPONSE_CHANNELS
env vars, matching Discord's existing mention gating pattern.

- MATTERMOST_REQUIRE_MENTION=false: respond to all channel messages
- MATTERMOST_FREE_RESPONSE_CHANNELS=id1,id2: specific channels where
  bot responds without @mention even when require_mention is true
- DMs always respond regardless of mention settings
- @mention is now stripped from message text (clean agent input)

7 new tests for mention gating, free-response channels, DM bypass,
and mention stripping. Updated existing test for mention stripping.

Docs: updated mattermost.md with Mention Behavior section,
environment-variables.md with new vars, config.py with metadata.

website/docs/reference/environment-variables.md

@@ -200,6 +200,8 @@ For native Anthropic auth, Hermes prefers Claude Code's own credential files whe
 | `MATTERMOST_TOKEN` | Bot token or personal access token for Mattermost |
 | `MATTERMOST_ALLOWED_USERS` | Comma-separated Mattermost user IDs allowed to message the bot |
 | `MATTERMOST_HOME_CHANNEL` | Channel ID for proactive message delivery (cron, notifications) |
+| `MATTERMOST_REQUIRE_MENTION` | Require `@mention` in channels (default: `true`). Set to `false` to respond to all messages. |
+| `MATTERMOST_FREE_RESPONSE_CHANNELS` | Comma-separated channel IDs where bot responds without `@mention` |
 | `MATTERMOST_REPLY_MODE` | Reply style: `thread` (threaded replies) or `off` (flat messages, default) |
 | `MATRIX_HOMESERVER` | Matrix homeserver URL (e.g. `https://matrix.org`) |
 | `MATRIX_ACCESS_TOKEN` | Matrix access token for bot authentication |