feat: add Mattermost channel support

Add Mattermost as a supported messaging channel with bot API and WebSocket integration. Includes channel state tracking (tint, summary, details), multi-account support, and delivery target routing. Update documentation and tests to include Mattermost alongside existing channels.

extensions/mattermost/clawdbot.plugin.json

@@ -0,0 +1,11 @@
+{
+  "id": "mattermost",
+  "channels": [
+    "mattermost"
+  ],
+  "configSchema": {
+    "type": "object",
+    "additionalProperties": false,
+    "properties": {}
+  }
+}