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.

extensions/mattermost/src/onboarding-helpers.ts

@@ -0,0 +1,42 @@
+import type { ClawdbotConfig, WizardPrompter } from "clawdbot/plugin-sdk";
+import { DEFAULT_ACCOUNT_ID, normalizeAccountId } from "clawdbot/plugin-sdk";
+
+type PromptAccountIdParams = {
+  cfg: ClawdbotConfig;
+  prompter: WizardPrompter;
+  label: string;
+  currentId?: string;
+  listAccountIds: (cfg: ClawdbotConfig) => string[];
+  defaultAccountId: string;
+};
+
+export async function promptAccountId(params: PromptAccountIdParams): Promise<string> {
+  const existingIds = params.listAccountIds(params.cfg);
+  const initial = params.currentId?.trim() || params.defaultAccountId || DEFAULT_ACCOUNT_ID;
+  const choice = (await params.prompter.select({
+    message: `${params.label} account`,
+    options: [
+      ...existingIds.map((id) => ({
+        value: id,
+        label: id === DEFAULT_ACCOUNT_ID ? "default (primary)" : id,
+      })),
+      { value: "__new__", label: "Add a new account" },
+    ],
+    initialValue: initial,
+  })) as string;
+
+  if (choice !== "__new__") return normalizeAccountId(choice);
+
+  const entered = await params.prompter.text({
+    message: `New ${params.label} account id`,
+    validate: (value) => (value?.trim() ? undefined : "Required"),
+  });
+  const normalized = normalizeAccountId(String(entered));
+  if (String(entered).trim() !== normalized) {
+    await params.prompter.note(
+      `Normalized account id to "${normalized}".`,
+      `${params.label} account`,
+    );
+  }
+  return normalized;
+}