events: metadata rides as headers, not in the body (ADR 0047)

emit stamps the ADR 0047 headers — x-event-id, x-source, x-node, x-time,
content-type, and optional x-causation-id / x-schema — and publishes the
body as only the domain payload. on() reconstructs the Event from those
headers. Event gains id (the x-event-id a consumer dedups on) plus the
optional causation/schema. EventHeaders joins the contracts spine.

Supersedes the first cut that carried source/node/time in the body.

Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF

src/messaging/index.ts

@@ -3,9 +3,9 @@
 // binding is provided by the runtime that hosts a module's code — the sdk defines the contract so
 // module code, the tool runtime and provisioners all speak it the same way.
 
-import type { Envelope } from "../contracts/index.js";
+import type { Envelope, EventHeaders } from "../contracts/index.js";
 
-export type { Envelope };
+export type { Envelope, EventHeaders };
 
 /** A request/reply call and a publish/subscribe surface over the mesh broker. */
 export interface Broker {