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/api-server/routes-messages.ts

@@ -1,6 +1,6 @@
 import log from "electron-log/main.js";
 import { agentMessageInputSchema } from "@shared/contracts";
-import { dispatchAgentMessage } from "../message-dispatch";
+import { dispatchAgentMessage } from "../messaging/dispatch";
 import { requireParam, type AgentApiServer } from "./server-core";
 
 export function registerMessageRoutes(server: AgentApiServer) {