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/core/src/store.test.ts

@@ -4301,7 +4301,7 @@ Task with acceptance criteria
 
       const { execSync } = await import("node:child_process");
       try {
-        execSync(`git checkout -b kb/${task.id.toLowerCase()}`, { cwd: rootDir, stdio: "pipe" });
+        execSync(`git checkout -b fusion/${task.id.toLowerCase()}`, { cwd: rootDir, stdio: "pipe" });
         execSync('git commit --allow-empty -m "test commit"', { cwd: rootDir, stdio: "pipe" });
         execSync("git checkout main || git checkout master", { cwd: rootDir, stdio: "pipe" });
       } catch {
@@ -4333,7 +4333,7 @@ Task with acceptance criteria
       // Create branch for merge
       const { execSync } = await import("node:child_process");
       try {
-        execSync(`git checkout -b kb/${task.id.toLowerCase()}`, { cwd: rootDir, stdio: "pipe" });
+        execSync(`git checkout -b fusion/${task.id.toLowerCase()}`, { cwd: rootDir, stdio: "pipe" });
         execSync('git commit --allow-empty -m "test commit"', { cwd: rootDir, stdio: "pipe" });
         execSync("git checkout main || git checkout master", { cwd: rootDir, stdio: "pipe" });
       } catch {