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/scheduler.ts

@@ -326,7 +326,7 @@ export class Scheduler {
     for (const depId of task.dependencies) {
       const dep = allTasks.find((t) => t.id === depId);
       if (dep && dep.column === "in-review" && dep.worktree) {
-        return `fusion/${dep.id.toLowerCase()}`;
+        return dep.branch || `fusion/${dep.id.toLowerCase()}`;
       }
     }
 
@@ -334,7 +334,7 @@ export class Scheduler {
     if (task.blockedBy) {
       const blocker = allTasks.find((t) => t.id === task.blockedBy);
       if (blocker && blocker.column === "in-review" && blocker.worktree) {
-        return `kb/${blocker.id.toLowerCase()}`;
+        return blocker.branch || `fusion/${blocker.id.toLowerCase()}`;
       }
     }