fix(types): correct GitHubOperations interface and test type assertions

- Tighten GitHubOperations.findPrForBranch and mergePr param types to
  match the literal unions in FindPrParams/MergePrParams, fixing the
  GitHubClient assignability error in dashboard.ts and serve.ts
- Cast MockStore as unknown as TaskStore at mesh-routes.test.ts call
  site to satisfy TaskStore shape without implementing 117 methods
- Double-cast AgentGenerationSession via unknown in agent-generation.test.ts
  to silence the unsafe conversion diagnostic

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/dashboard/src/__tests__/mesh-routes.test.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { EventEmitter } from "node:events";
-import type { Task } from "@fusion/core";
+import type { Task, TaskStore } from "@fusion/core";
 import { request } from "../test-request.js";
 import { createServer } from "../server.js";
 
@@ -137,7 +137,7 @@ describe("POST /api/mesh/sync", () => {
     });
 
     const store = new MockStore();
-    app = createServer(store);
+    app = createServer(store as unknown as TaskStore);
   });
 
   it("should merge peers and return sync response", async () => {