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.

pyproject.toml

@@ -46,6 +46,7 @@ dev = ["pytest", "pytest-asyncio", "pytest-xdist", "mcp>=1.2.0"]
 messaging = ["python-telegram-bot>=20.0", "discord.py[voice]>=2.0", "aiohttp>=3.9.0", "slack-bolt>=1.18.0", "slack-sdk>=3.27.0"]
 cron = ["croniter"]
 slack = ["slack-bolt>=1.18.0", "slack-sdk>=3.27.0"]
+matrix = ["matrix-nio[e2e]>=0.24.0"]
 cli = ["simple-term-menu"]
 tts-premium = ["elevenlabs"]
 voice = ["sounddevice>=0.4.6", "numpy>=1.24.0"]