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/config.py

@@ -40,6 +40,8 @@ class Platform(Enum):
     WHATSAPP = "whatsapp"
     SLACK = "slack"
     SIGNAL = "signal"
+    MATTERMOST = "mattermost"
+    MATRIX = "matrix"
     HOMEASSISTANT = "homeassistant"
     EMAIL = "email"
     SMS = "sms"
@@ -442,6 +444,8 @@ def load_gateway_config() -> GatewayConfig:
         Platform.TELEGRAM: "TELEGRAM_BOT_TOKEN",
         Platform.DISCORD: "DISCORD_BOT_TOKEN",
         Platform.SLACK: "SLACK_BOT_TOKEN",
+        Platform.MATTERMOST: "MATTERMOST_TOKEN",
+        Platform.MATRIX: "MATRIX_ACCESS_TOKEN",
     }
     for platform, pconfig in config.platforms.items():
         if not pconfig.enabled:
@@ -535,6 +539,53 @@ def _apply_env_overrides(config: GatewayConfig) -> None:
                 name=os.getenv("SIGNAL_HOME_CHANNEL_NAME", "Home"),
             )
 
+    # Mattermost
+    mattermost_token = os.getenv("MATTERMOST_TOKEN")
+    if mattermost_token:
+        mattermost_url = os.getenv("MATTERMOST_URL", "")
+        if not mattermost_url:
+            logger.warning("MATTERMOST_TOKEN set but MATTERMOST_URL is missing")
+        if Platform.MATTERMOST not in config.platforms:
+            config.platforms[Platform.MATTERMOST] = PlatformConfig()
+        config.platforms[Platform.MATTERMOST].enabled = True
+        config.platforms[Platform.MATTERMOST].token = mattermost_token
+        config.platforms[Platform.MATTERMOST].extra["url"] = mattermost_url
+        mattermost_home = os.getenv("MATTERMOST_HOME_CHANNEL")
+        if mattermost_home:
+            config.platforms[Platform.MATTERMOST].home_channel = HomeChannel(
+                platform=Platform.MATTERMOST,
+                chat_id=mattermost_home,
+                name=os.getenv("MATTERMOST_HOME_CHANNEL_NAME", "Home"),
+            )
+
+    # Matrix
+    matrix_token = os.getenv("MATRIX_ACCESS_TOKEN")
+    matrix_homeserver = os.getenv("MATRIX_HOMESERVER", "")
+    if matrix_token or os.getenv("MATRIX_PASSWORD"):
+        if not matrix_homeserver:
+            logger.warning("MATRIX_ACCESS_TOKEN/MATRIX_PASSWORD set but MATRIX_HOMESERVER is missing")
+        if Platform.MATRIX not in config.platforms:
+            config.platforms[Platform.MATRIX] = PlatformConfig()
+        config.platforms[Platform.MATRIX].enabled = True
+        if matrix_token:
+            config.platforms[Platform.MATRIX].token = matrix_token
+        config.platforms[Platform.MATRIX].extra["homeserver"] = matrix_homeserver
+        matrix_user = os.getenv("MATRIX_USER_ID", "")
+        if matrix_user:
+            config.platforms[Platform.MATRIX].extra["user_id"] = matrix_user
+        matrix_password = os.getenv("MATRIX_PASSWORD", "")
+        if matrix_password:
+            config.platforms[Platform.MATRIX].extra["password"] = matrix_password
+        matrix_e2ee = os.getenv("MATRIX_ENCRYPTION", "").lower() in ("true", "1", "yes")
+        config.platforms[Platform.MATRIX].extra["encryption"] = matrix_e2ee
+        matrix_home = os.getenv("MATRIX_HOME_ROOM")
+        if matrix_home:
+            config.platforms[Platform.MATRIX].home_channel = HomeChannel(
+                platform=Platform.MATRIX,
+                chat_id=matrix_home,
+                name=os.getenv("MATRIX_HOME_ROOM_NAME", "Home"),
+            )
+
     # Home Assistant
     hass_token = os.getenv("HASS_TOKEN")
     if hass_token: