feat(FN-5007): complete Step 2 — add merger finalized-task guard

Fusion-Task-Id: FN-5007
Fusion-Task-Lineage: 934e5a11-898d-4623-93d3-1c49d13c5d4c

packages/core/src/types.ts

@@ -3350,6 +3350,8 @@ export interface MergeResult extends MergeDetails {
   branch: string;
   merged: boolean;
   noOp?: boolean;
+  ok?: true;
+  reason?: string;
   worktreeRemoved: boolean;
   branchDeleted: boolean;
   error?: string;