fix(FN-834): fix branch prefix drift, add merger branch guard, and fix test OOM

- Fix resolveBaseBranch to use stored branch name and consistent fusion/ prefix
  for both explicit deps and blockedBy paths (was using kb/ for blockedBy)
- Add main branch checkout verification in merger before squash merge to prevent
  feature code from landing on wrong branch lineage
- Align all branch prefix references from stale kb/ to fusion/ across executor,
  merger, store, and routes
- Fix executor test OOM by mocking merger fully, adding fake timers to retry
  tests, and switching vitest pool to vmThreads
- Update all test assertions to use fusion/ branch prefix

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

packages/engine/src/restart.integration.test.ts

@@ -38,6 +38,16 @@ vi.mock("node:fs", () => ({
 vi.mock("node:fs/promises", () => ({
   readFile: vi.fn().mockResolvedValue("# Task prompt content"),
 }));
+vi.mock("@mariozechner/pi-coding-agent", () => {
+  const mockSessionManager = {};
+  return {
+    SessionManager: {
+      create: vi.fn().mockReturnValue(mockSessionManager),
+      open: vi.fn().mockReturnValue(mockSessionManager),
+      inMemory: vi.fn().mockReturnValue(mockSessionManager),
+    },
+  };
+});
 
 import { TaskExecutor } from "./executor.js";
 import { TriageProcessor } from "./triage.js";