feat(FN-709): store branch name on tasks for reliable merge recovery

- Add 'branch' field to Task and ArchivedTaskEntry types with DB migration
- Executor stores branch name on task after worktree assignment
- Merger reads branch from task metadata instead of relying on worktree state
- Implement non-destructive conflict recovery in prepareForTask with reset/clean
- Add tests for branch storage, merger branch reading, and worktree recovery

packages/engine/src/merger.ts

@@ -557,7 +557,7 @@ export async function aiMergeTask(
     );
   }
 
-  const branch = `kb/${taskId.toLowerCase()}`;
+  const branch = task.branch || `kb/${taskId.toLowerCase()}`;
   const worktreePath = task.worktree;
   const result: MergeResult = {
     task,