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/cron/isolated-agent.ts

@@ -160,7 +160,8 @@ function resolveDeliveryTarget(
       | "discord"
       | "slack"
       | "signal"
-      | "imessage";
+      | "imessage"
+      | "msteams";
     to?: string;
   },
 ) {

src/cron/types.ts

@@ -23,7 +23,8 @@ export type CronPayload =
         | "discord"
         | "slack"
         | "signal"
-        | "imessage";
+        | "imessage"
+        | "msteams";
       to?: string;
       bestEffortDeliver?: boolean;
     };