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/task-quality-gates.ts

@@ -9,7 +9,7 @@ import log from "electron-log/main.js";
 import type { DesktopEvent } from "@shared/types";
 import type { AppDatabase } from "@main/database";
 import type { NotificationOptions } from "@main/notification-service";
-import type { dispatchAgentMessage } from "@main/message-dispatch";
+import type { dispatchAgentMessage } from "@main/messaging/dispatch";
 import type { resolveApprovalDecision } from "@main/approval-policy";
 import { validateTaskRubric } from "@main/task-rubrics";