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.

src/config/zod-schema.agent-runtime.ts

@@ -20,6 +20,7 @@ export const HeartbeatSchema = z
         z.literal("telegram"),
         z.literal("discord"),
         z.literal("slack"),
+        z.literal("mattermost"),
         z.literal("msteams"),
         z.literal("signal"),
         z.literal("imessage"),