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/plugin-sdk/index.ts

@@ -81,7 +81,6 @@ export type {
 export {
   DiscordConfigSchema,
   IMessageConfigSchema,
-  MattermostConfigSchema,
   MSTeamsConfigSchema,
   SignalConfigSchema,
   SlackConfigSchema,
@@ -121,7 +120,6 @@ export {
   resolveBlueBubblesGroupRequireMention,
   resolveDiscordGroupRequireMention,
   resolveIMessageGroupRequireMention,
-  resolveMattermostGroupRequireMention,
   resolveSlackGroupRequireMention,
   resolveTelegramGroupRequireMention,
   resolveWhatsAppGroupRequireMention,
@@ -241,21 +239,6 @@ export {
 } from "../channels/plugins/normalize/slack.js";
 export { buildSlackThreadingToolContext } from "../slack/threading-tool-context.js";
 
-// Channel: Mattermost
-export {
-  listEnabledMattermostAccounts,
-  listMattermostAccountIds,
-  resolveDefaultMattermostAccountId,
-  resolveMattermostAccount,
-  type ResolvedMattermostAccount,
-} from "../mattermost/accounts.js";
-export { normalizeMattermostBaseUrl } from "../mattermost/client.js";
-export { mattermostOnboardingAdapter } from "../channels/plugins/onboarding/mattermost.js";
-export {
-  looksLikeMattermostTargetId,
-  normalizeMattermostMessagingTarget,
-} from "../channels/plugins/normalize/mattermost.js";
-
 // Channel: Telegram
 export {
   listTelegramAccountIds,