feat(FN-4371): complete Step 2 — executor no-commits guard flag

Fusion-Task-Id: FN-4371
Fusion-Task-Lineage: 4175ba4c-b7cb-4f3a-87c5-75e51f06c0ff

packages/engine/src/__tests__/executor-task-done-invariant.test.ts

@@ -102,6 +102,37 @@ describe("FN-4114 fn_task_done invariants", () => {
     expect(store.moveTask).toHaveBeenCalledWith("FN-4114", "todo", { preserveProgress: true });
   });
 
+  it("FN-4114 allows no-commit completion when noCommitsExpected is true", async () => {
+    const { store, tool } = await setup({ noCommitsExpected: true });
+    mockedExecSync.mockImplementation((cmd: string) => {
+      if (cmd.includes("rev-parse --show-toplevel")) return Buffer.from("/repo/.worktrees/swift-falcon\n");
+      if (cmd.includes("rev-parse --abbrev-ref HEAD")) return Buffer.from("fusion/fn-4114\n");
+      if (cmd.includes("rev-list --count")) return Buffer.from("0\n");
+      if (cmd.includes("rev-parse HEAD")) return Buffer.from("def456\n");
+      return Buffer.from("");
+    });
+
+    const result = await tool.execute("id", {});
+    expect(result.content[0].text).toContain("Task marked complete");
+    expect(store.updateStep).toHaveBeenCalled();
+    const revListCalled = mockedExecSync.mock.calls.some(([cmd]) => String(cmd).includes("rev-list --count"));
+    expect(revListCalled).toBe(false);
+  });
+
+  it("FN-4114 still refuses wrong_branch even when noCommitsExpected is true", async () => {
+    const { store, tool } = await setup({ noCommitsExpected: true });
+    mockedExecSync.mockImplementation((cmd: string) => {
+      if (cmd.includes("rev-parse --show-toplevel")) return Buffer.from("/repo/.worktrees/swift-falcon\n");
+      if (cmd.includes("rev-parse --abbrev-ref HEAD")) return Buffer.from("main\n");
+      if (cmd.includes("rev-parse HEAD")) return Buffer.from("def456\n");
+      return Buffer.from("");
+    });
+
+    const result = await tool.execute("id", {});
+    expect(result.content[0].text).toContain("fn_task_done refused: wrong_branch");
+    expect(store.moveTask).toHaveBeenCalledWith("FN-4114", "todo", { preserveProgress: true });
+  });
+
   it("FN-4114 allows fn_task_done on valid worktree/branch/commit state", async () => {
     const { store, tool } = await setup();
     const result = await tool.execute("id", {});

packages/engine/src/executor.ts

@@ -4688,6 +4688,11 @@ export class TaskExecutor {
       };
     }
 
+    if (task.noCommitsExpected === true) {
+      executorLog.log(`${task.id}: fn_task_done no_commits guard skipped (noCommitsExpected=true)`);
+      return { ok: true };
+    }
+
     const baseRef = await this.resolveDiffBaseRef(worktreePath, task.baseCommitSha);
     if (!baseRef) {
       executorLog.warn(`${task.id}: unable to resolve diff base for invariant commit-count check; skipping no_commits guard`);