Move message-*.ts into messaging/ subdir (N10)

Consolidate the two runtime messaging modules under src/main/messaging/. Storage stays in database/messages.ts — that is the SQL layer and belongs with the other DB modules; messaging/ covers the dispatch/normalize runtime around it.

- message-dispatch.ts -> messaging/dispatch.ts
- message-normalize.ts -> messaging/normalize.ts
- add messaging/index.ts barrel for future consumers (existing call sites import the concrete module path directly)
- 10 call sites updated across src/main and tests

src/main/orchestration/heartbeat.ts

@@ -4,7 +4,7 @@ import type { AgentRecord, ConnectorRecord } from "@shared/types";
 import type { OrchestrationDependencies } from "./types";
 import type { RunDispatchParams } from "./turn-context";
 import { pickModelForAgent } from "../model-dispatch";
-import { HUMAN_SENDER_ID } from "../message-dispatch";
+import { HUMAN_SENDER_ID } from "../messaging/dispatch";
 
 /**
  * Every reason a heartbeat attempt can be rejected before a run is