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.

hermes_cli/gateway.py

@@ -1001,6 +1001,64 @@ _PLATFORMS = [
              "help": "Paste your member ID from step 7 above."},
         ],
     },
+    {
+        "key": "matrix",
+        "label": "Matrix",
+        "emoji": "🔐",
+        "token_var": "MATRIX_ACCESS_TOKEN",
+        "setup_instructions": [
+            "1. Works with any Matrix homeserver (self-hosted Synapse/Conduit/Dendrite or matrix.org)",
+            "2. Create a bot user on your homeserver, or use your own account",
+            "3. Get an access token: Element → Settings → Help & About → Access Token",
+            "   Or via API: curl -X POST https://your-server/_matrix/client/v3/login \\",
+            "     -d '{\"type\":\"m.login.password\",\"user\":\"@bot:server\",\"password\":\"...\"}'",
+            "4. Alternatively, provide user ID + password and Hermes will log in directly",
+            "5. For E2EE: set MATRIX_ENCRYPTION=true (requires pip install 'matrix-nio[e2e]')",
+            "6. To find your user ID: it's @username:your-server (shown in Element profile)",
+        ],
+        "vars": [
+            {"name": "MATRIX_HOMESERVER", "prompt": "Homeserver URL (e.g. https://matrix.example.org)", "password": False,
+             "help": "Your Matrix homeserver URL. Works with any self-hosted instance."},
+            {"name": "MATRIX_ACCESS_TOKEN", "prompt": "Access token (leave empty to use password login instead)", "password": True,
+             "help": "Paste your access token, or leave empty and provide user ID + password below."},
+            {"name": "MATRIX_USER_ID", "prompt": "User ID (@bot:server — required for password login)", "password": False,
+             "help": "Full Matrix user ID, e.g. @hermes:matrix.example.org"},
+            {"name": "MATRIX_ALLOWED_USERS", "prompt": "Allowed user IDs (comma-separated, e.g. @you:server)", "password": False,
+             "is_allowlist": True,
+             "help": "Matrix user IDs who can interact with the bot."},
+            {"name": "MATRIX_HOME_ROOM", "prompt": "Home room ID (for cron/notification delivery, or empty to set later with /set-home)", "password": False,
+             "help": "Room ID (e.g. !abc123:server) for delivering cron results and notifications."},
+        ],
+    },
+    {
+        "key": "mattermost",
+        "label": "Mattermost",
+        "emoji": "💬",
+        "token_var": "MATTERMOST_TOKEN",
+        "setup_instructions": [
+            "1. In Mattermost: Integrations → Bot Accounts → Add Bot Account",
+            "   (System Console → Integrations → Bot Accounts must be enabled)",
+            "2. Give it a username (e.g. hermes) and copy the bot token",
+            "3. Works with any self-hosted Mattermost instance — enter your server URL",
+            "4. To find your user ID: click your avatar (top-left) → Profile",
+            "   Your user ID is displayed there — click it to copy.",
+            "   ⚠ This is NOT your username — it's a 26-character alphanumeric ID.",
+            "5. To get a channel ID: click the channel name → View Info → copy the ID",
+        ],
+        "vars": [
+            {"name": "MATTERMOST_URL", "prompt": "Server URL (e.g. https://mm.example.com)", "password": False,
+             "help": "Your Mattermost server URL. Works with any self-hosted instance."},
+            {"name": "MATTERMOST_TOKEN", "prompt": "Bot token", "password": True,
+             "help": "Paste the bot token from step 2 above."},
+            {"name": "MATTERMOST_ALLOWED_USERS", "prompt": "Allowed user IDs (comma-separated)", "password": False,
+             "is_allowlist": True,
+             "help": "Your Mattermost user ID from step 4 above."},
+            {"name": "MATTERMOST_HOME_CHANNEL", "prompt": "Home channel ID (for cron/notification delivery, or empty to set later with /set-home)", "password": False,
+             "help": "Channel ID where Hermes delivers cron results and notifications."},
+            {"name": "MATTERMOST_REPLY_MODE", "prompt": "Reply mode — 'off' for flat messages, 'thread' for threaded replies (default: off)", "password": False,
+             "help": "off = flat channel messages, thread = replies nest under your message."},
+        ],
+    },
     {
         "key": "whatsapp",
         "label": "WhatsApp",
@@ -1100,6 +1158,16 @@ def _platform_status(platform: dict) -> str:
         if any([val, pwd, imap, smtp]):
             return "partially configured"
         return "not configured"
+    if platform.get("key") == "matrix":
+        homeserver = get_env_value("MATRIX_HOMESERVER")
+        password = get_env_value("MATRIX_PASSWORD")
+        if (val or password) and homeserver:
+            e2ee = get_env_value("MATRIX_ENCRYPTION")
+            suffix = " + E2EE" if e2ee and e2ee.lower() in ("true", "1", "yes") else ""
+            return f"configured{suffix}"
+        if val or password or homeserver:
+            return "partially configured"
+        return "not configured"
     if val:
         return "configured"
     return "not configured"