test(FN-4958): complete Step 4 — reliability interaction backstop

Fusion-Task-Id: FN-4958
Fusion-Task-Lineage: 2f1bc742-14e0-4fdd-a980-00051fc3323c

packages/engine/src/__tests__/reliability-interactions/auto-prerebase-interactions.test.ts

@@ -0,0 +1,39 @@
+import { describe, expect, it } from "vitest";
+import { decideAutoPrerebase } from "../../merger-auto-prerebase.js";
+
+describe("auto-prerebase reliability interactions", () => {
+  it("defers when worktrunk is enabled", () => {
+    const decision = decideAutoPrerebase({
+      settings: { prerebaseAutoEnabled: true, prerebaseHotFiles: ["AGENTS.md"], prerebaseDivergenceThreshold: 50 } as any,
+      baseCommitSha: "abc",
+      commitsBehind: 100,
+      changedFiles: ["AGENTS.md"],
+      worktrunkEnabled: true,
+    });
+    expect(decision.fire).toBe(false);
+    expect(decision.reason).toBe("worktrunk-deferred");
+  });
+
+  it("hot-file trigger has precedence over threshold and remains compatible with smart-prefer-main strategy", () => {
+    const decision = decideAutoPrerebase({
+      settings: { prerebaseAutoEnabled: true, prerebaseHotFiles: ["packages/engine/src/merger.ts"], prerebaseDivergenceThreshold: 1 } as any,
+      baseCommitSha: "abc",
+      commitsBehind: 999,
+      changedFiles: ["packages/engine/src/merger.ts"],
+      worktrunkEnabled: false,
+    });
+    expect(decision.reason).toBe("hot-file");
+    expect(decision.fire).toBe(true);
+  });
+
+  it("no-divergence path preserves already-on-main/fast-path behavior", () => {
+    const decision = decideAutoPrerebase({
+      settings: { prerebaseAutoEnabled: true, prerebaseHotFiles: ["AGENTS.md"], prerebaseDivergenceThreshold: 50 } as any,
+      baseCommitSha: "abc",
+      commitsBehind: 0,
+      changedFiles: [],
+      worktrunkEnabled: false,
+    });
+    expect(decision.reason).toBe("no-divergence");
+  });
+});