feat: add Mattermost and Matrix gateway adapters

Add support for Mattermost (self-hosted Slack alternative) and Matrix
(federated messaging protocol) as messaging platforms.

Mattermost adapter:
- REST API v4 client for posts, files, channels, typing indicators
- WebSocket listener for real-time 'posted' events with reconnect backoff
- Thread support via root_id
- File upload/download with auth-aware caching
- Dedup cache (5min TTL, 2000 entries)
- Full self-hosted instance support

Matrix adapter:
- matrix-nio AsyncClient with sync loop
- Dual auth: access token or user_id + password
- Optional E2EE via matrix-nio[e2e] (libolm)
- Thread support via m.thread (MSC3440)
- Reply support via m.in_reply_to with fallback stripping
- Media upload/download via mxc:// URLs (authenticated v1.11+ endpoint)
- Auto-join on room invite
- DM detection via m.direct account data with sync fallback
- Markdown to HTML conversion

Fixes applied over original PR #1225 by @cyb0rgk1tty:
- Mattermost: add timeout to file downloads, wrap API helpers in
  try/except for network errors, download incoming files immediately
  with auth headers instead of passing auth-required URLs
- Matrix: use authenticated media endpoint (/_matrix/client/v1/media/),
  robust m.direct cache with sync fallback, prefer aiohttp over httpx

Install Matrix support: pip install 'hermes-agent[matrix]'
Mattermost needs no extra deps (uses aiohttp).

Salvaged from PR #1225 by @cyb0rgk1tty with fixes.

gateway/run.py

@@ -1147,6 +1147,20 @@ class GatewayRunner:
                 return None
             return DingTalkAdapter(config)
 
+        elif platform == Platform.MATTERMOST:
+            from gateway.platforms.mattermost import MattermostAdapter, check_mattermost_requirements
+            if not check_mattermost_requirements():
+                logger.warning("Mattermost: MATTERMOST_TOKEN or MATTERMOST_URL not set, or aiohttp missing")
+                return None
+            return MattermostAdapter(config)
+
+        elif platform == Platform.MATRIX:
+            from gateway.platforms.matrix import MatrixAdapter, check_matrix_requirements
+            if not check_matrix_requirements():
+                logger.warning("Matrix: matrix-nio not installed or credentials not set. Run: pip install 'matrix-nio[e2e]'")
+                return None
+            return MatrixAdapter(config)
+
         return None
     
     def _is_user_authorized(self, source: SessionSource) -> bool:
@@ -1178,6 +1192,8 @@ class GatewayRunner:
             Platform.SIGNAL: "SIGNAL_ALLOWED_USERS",
             Platform.EMAIL: "EMAIL_ALLOWED_USERS",
             Platform.SMS: "SMS_ALLOWED_USERS",
+            Platform.MATTERMOST: "MATTERMOST_ALLOWED_USERS",
+            Platform.MATRIX: "MATRIX_ALLOWED_USERS",
         }
         platform_allow_all_map = {
             Platform.TELEGRAM: "TELEGRAM_ALLOW_ALL_USERS",
@@ -1187,6 +1203,8 @@ class GatewayRunner:
             Platform.SIGNAL: "SIGNAL_ALLOW_ALL_USERS",
             Platform.EMAIL: "EMAIL_ALLOW_ALL_USERS",
             Platform.SMS: "SMS_ALLOW_ALL_USERS",
+            Platform.MATTERMOST: "MATTERMOST_ALLOW_ALL_USERS",
+            Platform.MATRIX: "MATRIX_ALLOW_ALL_USERS",
         }
 
         # Per-platform allow-all flag (e.g., DISCORD_ALLOW_ALL_USERS=true)