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.

hermes_cli/config.py

@@ -817,6 +817,20 @@ OPTIONAL_ENV_VARS = {
         "password": False,
         "category": "messaging",
     },
+    "MATTERMOST_REQUIRE_MENTION": {
+        "description": "Require @mention in Mattermost channels (default: true). Set to false to respond to all messages.",
+        "prompt": "Require @mention in channels",
+        "url": None,
+        "password": False,
+        "category": "messaging",
+    },
+    "MATTERMOST_FREE_RESPONSE_CHANNELS": {
+        "description": "Comma-separated Mattermost channel IDs where bot responds without @mention",
+        "prompt": "Free-response channel IDs (comma-separated)",
+        "url": None,
+        "password": False,
+        "category": "messaging",
+    },
     "MATRIX_HOMESERVER": {
         "description": "Matrix homeserver URL (e.g. https://matrix.example.org)",
         "prompt": "Matrix homeserver URL",