refactor: extract mattermost channel plugin to extension

Move mattermost channel implementation from core to extensions/mattermost plugin. Extract config schema, group mentions, normalize utilities, and all mattermost-specific logic (accounts, client, monitor, probe, send) into the extension. Update imports to use plugin SDK and local modules. Add channel metadata directly in plugin definition instead of using getChatChannelMeta. Update package.json with channel and install configuration.

src/infra/outbound/deliver.ts

@@ -6,7 +6,6 @@ import type { ChannelOutboundAdapter } from "../../channels/plugins/types.js";
 import type { ClawdbotConfig } from "../../config/config.js";
 import type { sendMessageDiscord } from "../../discord/send.js";
 import type { sendMessageIMessage } from "../../imessage/send.js";
-import type { sendMessageMattermost } from "../../mattermost/send.js";
 import { markdownToSignalTextChunks, type SignalTextStyleRange } from "../../signal/format.js";
 import { sendMessageSignal } from "../../signal/send.js";
 import type { sendMessageSlack } from "../../slack/send.js";
@@ -29,12 +28,18 @@ type SendMatrixMessage = (
   opts?: { mediaUrl?: string; replyToId?: string; threadId?: string; timeoutMs?: number },
 ) => Promise<{ messageId: string; roomId: string }>;
 
+type SendMattermostMessage = (
+  to: string,
+  text: string,
+  opts?: { accountId?: string; mediaUrl?: string; replyToId?: string },
+) => Promise<{ messageId: string; channelId: string }>;
+
 export type OutboundSendDeps = {
   sendWhatsApp?: typeof sendMessageWhatsApp;
   sendTelegram?: typeof sendMessageTelegram;
   sendDiscord?: typeof sendMessageDiscord;
   sendSlack?: typeof sendMessageSlack;
-  sendMattermost?: typeof sendMessageMattermost;
+  sendMattermost?: SendMattermostMessage;
   sendSignal?: typeof sendMessageSignal;
   sendIMessage?: typeof sendMessageIMessage;
   sendMatrix?: SendMatrixMessage;