feat(matrix): Matrix messages panel in mesh dashboard

[mdheller]

Summary

• _matrix_config() — reads MATRIX_HOMESERVER_URL / MATRIX_ACCESS_TOKEN / MATRIX_BOT_ROOM_ID from env, falling back to ~/.config/sourceos/matrix.yaml (same key-set the bridge uses)
• _matrix_recent_messages(n) — calls GET /_matrix/client/v3/rooms/{room}/messages?dir=b&limit={n} via urllib.request (3 s timeout), filters m.text events, reverses to newest-last; returns [] silently on any error so the dashboard stays healthy when Matrix is unconfigured
• gather_state() extended with matrix_messages (8 most-recent) and matrix_room_id
• Dashboard: new Matrix Room card below Runbooks — shows sender, HH:MM timestamp, and up to 120 chars of body per message; empty state prompts to configure MATRIX_BOT_ROOM_ID
• GET /api/matrix/rooms — proxies /_matrix/client/v3/joined_rooms with the configured bearer token (5 s timeout); returns {"rooms": [], "error": "..."} on failure
• GET /api/matrix/messages — returns {"messages": [...]} for last 20 messages, used by future Lua/WezTerm pane consumers

Test plan

• python3 -m py_compile assets/sourceos/bin/turtle-mesh-serve passes (verified before commit)
• With Matrix unconfigured: dashboard loads, Matrix card shows "No messages — configure MATRIX_BOT_ROOM_ID", /api/matrix/rooms returns {"rooms":[],"error":"Matrix not configured"}
• With valid env vars set: card shows recent messages; /api/matrix/rooms returns joined room list; /api/matrix/messages returns up to 20 messages