feat(FN-3713): migrate WhatsApp plugin to Baileys pairing flow

- Add Baileys-based connection/auth modules and pairing route support for QR, pair code, status, and logout
- Refactor plugin entrypoint and split reply/auth/connection logic into focused modules
- Update plugin metadata/docs/settings schema to pairing-era configuration and remove legacy webhook key usage
- Add targeted tests for auth state, connection handling, reply flow, and updated schema expectations

Fusion-Task-Id: FN-3713

packages/dashboard/app/components/PluginManager.tsx

@@ -125,7 +125,7 @@ const BUILTIN_PLUGINS: BuiltinPlugin[] = [
   {
     id: "fusion-plugin-whatsapp-chat",
     name: "WhatsApp Chat",
-    description: "Connects WhatsApp Cloud webhooks to a Fusion agent conversation; requires Meta webhook and API credentials.",
+    description: "Pairs to WhatsApp Web (multi-device) with QR or pairing code, then bridges direct chats to a Fusion agent.",
     category: "integration",
     path: "./plugins/fusion-plugin-whatsapp-chat",
   },

packages/dashboard/app/components/__tests__/PluginManager.test.tsx

@@ -268,6 +268,7 @@ describe("PluginManager", () => {
     expect(screen.getByText("Droid Runtime")).toBeTruthy();
     expect(screen.getByText("Dependency Graph")).toBeTruthy();
     expect(screen.getByText("WhatsApp Chat")).toBeTruthy();
+    expect(screen.getByText(/Pairs to WhatsApp Web \(multi-device\) with QR or pairing code/i)).toBeTruthy();
   });
 
   it("renders built-in agent browser metadata-only entry when uninstalled", async () => {