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/src/runtime.ts

@@ -0,0 +1,14 @@
+import type { PluginRuntime } from "clawdbot/plugin-sdk";
+
+let runtime: PluginRuntime | null = null;
+
+export function setMattermostRuntime(next: PluginRuntime) {
+  runtime = next;
+}
+
+export function getMattermostRuntime(): PluginRuntime {
+  if (!runtime) {
+    throw new Error("Mattermost runtime not initialized");
+  }
+  return runtime;
+}