feat(msteams): add MS Teams provider skeleton

- Add Microsoft 365 Agents SDK packages (@microsoft/agents-hosting,
  @microsoft/agents-hosting-express, @microsoft/agents-hosting-extensions-teams)
- Add MSTeamsConfig type and zod schema
- Create src/msteams/ provider with monitor, token, send, probe
- Wire provider into gateway (server-providers.ts, server.ts)
- Add msteams to all provider type unions (hooks, queue, cron, etc.)
- Update implementation guide with new SDK and progress

src/msteams/send.ts

@@ -0,0 +1,25 @@
+import type { MSTeamsConfig } from "../config/types.js";
+import { getChildLogger } from "../logging.js";
+
+const log = getChildLogger({ name: "msteams:send" });
+
+export type SendMSTeamsMessageParams = {
+  cfg: MSTeamsConfig;
+  conversationId: string;
+  text: string;
+  serviceUrl: string;
+};
+
+export type SendMSTeamsMessageResult = {
+  ok: boolean;
+  messageId?: string;
+  error?: string;
+};
+
+export async function sendMessageMSTeams(
+  _params: SendMSTeamsMessageParams,
+): Promise<SendMSTeamsMessageResult> {
+  // TODO: Implement using CloudAdapter.continueConversationAsync
+  log.warn("sendMessageMSTeams not yet implemented");
+  return { ok: false, error: "not implemented" };
+}