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/setup.py

@@ -2518,6 +2518,119 @@ def setup_gateway(config: dict):
                     "   Set SLACK_ALLOW_ALL_USERS=true or GATEWAY_ALLOW_ALL_USERS=true only if you intentionally want open workspace access."
                 )
 
+    # ── Matrix ──
+    existing_matrix = get_env_value("MATRIX_ACCESS_TOKEN") or get_env_value("MATRIX_PASSWORD")
+    if existing_matrix:
+        print_info("Matrix: already configured")
+        if prompt_yes_no("Reconfigure Matrix?", False):
+            existing_matrix = None
+
+    if not existing_matrix and prompt_yes_no("Set up Matrix?", False):
+        print_info("Works with any Matrix homeserver (Synapse, Conduit, Dendrite, or matrix.org).")
+        print_info("   1. Create a bot user on your homeserver, or use your own account")
+        print_info("   2. Get an access token from Element, or provide user ID + password")
+        print()
+        homeserver = prompt("Homeserver URL (e.g. https://matrix.example.org)")
+        if homeserver:
+            save_env_value("MATRIX_HOMESERVER", homeserver.rstrip("/"))
+
+        print()
+        print_info("Auth: provide an access token (recommended), or user ID + password.")
+        token = prompt("Access token (leave empty for password login)", password=True)
+        if token:
+            save_env_value("MATRIX_ACCESS_TOKEN", token)
+            user_id = prompt("User ID (@bot:server — optional, will be auto-detected)")
+            if user_id:
+                save_env_value("MATRIX_USER_ID", user_id)
+            print_success("Matrix access token saved")
+        else:
+            user_id = prompt("User ID (@bot:server)")
+            if user_id:
+                save_env_value("MATRIX_USER_ID", user_id)
+            password = prompt("Password", password=True)
+            if password:
+                save_env_value("MATRIX_PASSWORD", password)
+                print_success("Matrix credentials saved")
+
+        if token or get_env_value("MATRIX_PASSWORD"):
+            # E2EE
+            print()
+            if prompt_yes_no("Enable end-to-end encryption (E2EE)?", False):
+                save_env_value("MATRIX_ENCRYPTION", "true")
+                print_success("E2EE enabled")
+                print_info("   Requires: pip install 'matrix-nio[e2e]'")
+
+            # Allowed users
+            print()
+            print_info("🔒 Security: Restrict who can use your bot")
+            print_info("   Matrix user IDs look like @username:server")
+            print()
+            allowed_users = prompt(
+                "Allowed user IDs (comma-separated, leave empty for open access)"
+            )
+            if allowed_users:
+                save_env_value("MATRIX_ALLOWED_USERS", allowed_users.replace(" ", ""))
+                print_success("Matrix allowlist configured")
+            else:
+                print_info(
+                    "⚠️  No allowlist set - anyone who can message the bot can use it!"
+                )
+
+            # Home room
+            print()
+            print_info("📬 Home Room: where Hermes delivers cron job results and notifications.")
+            print_info("   Room IDs look like !abc123:server (shown in Element room settings)")
+            print_info("   You can also set this later by typing /set-home in a Matrix room.")
+            home_room = prompt("Home room ID (leave empty to set later with /set-home)")
+            if home_room:
+                save_env_value("MATRIX_HOME_ROOM", home_room)
+
+    # ── Mattermost ──
+    existing_mattermost = get_env_value("MATTERMOST_TOKEN")
+    if existing_mattermost:
+        print_info("Mattermost: already configured")
+        if prompt_yes_no("Reconfigure Mattermost?", False):
+            existing_mattermost = None
+
+    if not existing_mattermost and prompt_yes_no("Set up Mattermost?", False):
+        print_info("Works with any self-hosted Mattermost instance.")
+        print_info("   1. In Mattermost: Integrations → Bot Accounts → Add Bot Account")
+        print_info("   2. Copy the bot token")
+        print()
+        mm_url = prompt("Mattermost server URL (e.g. https://mm.example.com)")
+        if mm_url:
+            save_env_value("MATTERMOST_URL", mm_url.rstrip("/"))
+        token = prompt("Bot token", password=True)
+        if token:
+            save_env_value("MATTERMOST_TOKEN", token)
+            print_success("Mattermost token saved")
+
+            # Allowed users
+            print()
+            print_info("🔒 Security: Restrict who can use your bot")
+            print_info("   To find your user ID: click your avatar → Profile")
+            print_info("   or use the API: GET /api/v4/users/me")
+            print()
+            allowed_users = prompt(
+                "Allowed user IDs (comma-separated, leave empty for open access)"
+            )
+            if allowed_users:
+                save_env_value("MATTERMOST_ALLOWED_USERS", allowed_users.replace(" ", ""))
+                print_success("Mattermost allowlist configured")
+            else:
+                print_info(
+                    "⚠️  No allowlist set - anyone who can message the bot can use it!"
+                )
+
+            # Home channel
+            print()
+            print_info("📬 Home Channel: where Hermes delivers cron job results and notifications.")
+            print_info("   To get a channel ID: click channel name → View Info → copy the ID")
+            print_info("   You can also set this later by typing /set-home in a Mattermost channel.")
+            home_channel = prompt("Home channel ID (leave empty to set later with /set-home)")
+            if home_channel:
+                save_env_value("MATTERMOST_HOME_CHANNEL", home_channel)
+
     # ── WhatsApp ──
     existing_whatsapp = get_env_value("WHATSAPP_ENABLED")
     if not existing_whatsapp and prompt_yes_no("Set up WhatsApp?", False):
@@ -2535,6 +2648,9 @@ def setup_gateway(config: dict):
         get_env_value("TELEGRAM_BOT_TOKEN")
         or get_env_value("DISCORD_BOT_TOKEN")
         or get_env_value("SLACK_BOT_TOKEN")
+        or get_env_value("MATTERMOST_TOKEN")
+        or get_env_value("MATRIX_ACCESS_TOKEN")
+        or get_env_value("MATRIX_PASSWORD")
         or get_env_value("WHATSAPP_ENABLED")
     )
     if any_messaging: