feat(FN-4518): complete Step 1 — add MergeDetails rebase base SHA

Fusion-Task-Id: FN-4518
Fusion-Task-Lineage: 48749dec-b2c9-484b-a13e-ffacc1fa9014

packages/core/src/__tests__/merge-details.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, it } from "vitest";
+
+import type { MergeDetails } from "../types";
+
+describe("MergeDetails", () => {
+  it("exposes optional rebaseBaseSha for rebase merge range attribution", () => {
+    const details: MergeDetails = {
+      commitSha: "abc123",
+      rebaseBaseSha: "def456",
+    };
+
+    expect(details.rebaseBaseSha).toBe("def456");
+  });
+});

packages/core/src/types.ts

@@ -1111,6 +1111,13 @@ export function buildResearchDocumentKey(runId: string): string {
 
 export interface MergeDetails {
   commitSha?: string;
+  /**
+   * When merger used rebase strategy (>=2 substantive commits), this is the
+   * parent SHA on the target branch before the cherry-pick chain. The canonical
+   * rebase display/audit range is `rebaseBaseSha..commitSha`.
+   * Unset for squash merges.
+   */
+  rebaseBaseSha?: string;
   filesChanged?: number;
   insertions?: number;
   deletions?: number;