feat(#378): Install matrix-bot-sdk and create MatrixService skeleton

- Add matrix-bot-sdk dependency to @mosaic/api
- Create MatrixService implementing IChatProvider interface
- Support connect/disconnect, message sending, thread management
- Parse @mosaic and !mosaic command prefixes
- Delegate commands to StitcherService (same flow as Discord)
- Add comprehensive unit tests with mocked MatrixClient (31 tests)
- Add Matrix env vars to .env.example

Refs #378

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -316,6 +316,22 @@ RATE_LIMIT_STORAGE=redis
 # multi-tenant isolation. Each Discord bot instance should be configured for
 # a single workspace.
 
+# ======================
+# Matrix Bridge (Optional)
+# ======================
+# Matrix bot integration for chat-based control via Matrix protocol
+# Requires a Matrix account with an access token for the bot user
+# MATRIX_HOMESERVER_URL=https://matrix.example.com
+# MATRIX_ACCESS_TOKEN=
+# MATRIX_BOT_USER_ID=@mosaic-bot:example.com
+# MATRIX_CONTROL_ROOM_ID=!roomid:example.com
+# MATRIX_WORKSPACE_ID=your-workspace-uuid
+#
+# SECURITY: MATRIX_WORKSPACE_ID must be a valid workspace UUID from your database.
+# All Matrix commands will execute within this workspace context for proper
+# multi-tenant isolation. Each Matrix bot instance should be configured for
+# a single workspace.
+
 # ======================
 # Orchestrator Configuration
 # ======================