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

tests/integration/thread-wait-heartbeat.spec.ts

@@ -5,7 +5,7 @@ import { afterEach, describe, expect, it } from "vitest";
 import { AppDatabase } from "../../src/main/database";
 import { createThreadIntegration } from "../../src/main/thread-integration";
 import { createCheckpointEngine } from "../../src/main/checkpoint-engine";
-import { dispatchAgentMessage, HUMAN_SENDER_ID } from "../../src/main/message-dispatch";
+import { dispatchAgentMessage, HUMAN_SENDER_ID } from "../../src/main/messaging/dispatch";
 
 const tempDirs: string[] = [];