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/mattermost/probe.ts

@@ -0,0 +1,70 @@
+import { normalizeMattermostBaseUrl, type MattermostUser } from "./client.js";
+
+export type MattermostProbe = {
+  ok: boolean;
+  status?: number | null;
+  error?: string | null;
+  elapsedMs?: number | null;
+  bot?: MattermostUser;
+};
+
+async function readMattermostError(res: Response): Promise<string> {
+  const contentType = res.headers.get("content-type") ?? "";
+  if (contentType.includes("application/json")) {
+    const data = (await res.json()) as { message?: string } | undefined;
+    if (data?.message) return data.message;
+    return JSON.stringify(data);
+  }
+  return await res.text();
+}
+
+export async function probeMattermost(
+  baseUrl: string,
+  botToken: string,
+  timeoutMs = 2500,
+): Promise<MattermostProbe> {
+  const normalized = normalizeMattermostBaseUrl(baseUrl);
+  if (!normalized) {
+    return { ok: false, error: "baseUrl missing" };
+  }
+  const url = `${normalized}/api/v4/users/me`;
+  const start = Date.now();
+  const controller = timeoutMs > 0 ? new AbortController() : undefined;
+  let timer: NodeJS.Timeout | null = null;
+  if (controller) {
+    timer = setTimeout(() => controller.abort(), timeoutMs);
+  }
+  try {
+    const res = await fetch(url, {
+      headers: { Authorization: `Bearer ${botToken}` },
+      signal: controller?.signal,
+    });
+    const elapsedMs = Date.now() - start;
+    if (!res.ok) {
+      const detail = await readMattermostError(res);
+      return {
+        ok: false,
+        status: res.status,
+        error: detail || res.statusText,
+        elapsedMs,
+      };
+    }
+    const bot = (await res.json()) as MattermostUser;
+    return {
+      ok: true,
+      status: res.status,
+      elapsedMs,
+      bot,
+    };
+  } catch (err) {
+    const message = err instanceof Error ? err.message : String(err);
+    return {
+      ok: false,
+      status: null,
+      error: message,
+      elapsedMs: Date.now() - start,
+    };
+  } finally {
+    if (timer) clearTimeout(timer);
+  }
+}