feat(FN-3968): add tests for task env inheritance and isolation in spawn/ch

Adds test coverage for task environment propagation through the spawn flow, covering child agents inheriting task env, workflow step task env, and spawn hook env isolation cases in `pi-create-fn-agent.test.ts`, with minor additions to `executor-step-session.test.ts` and `executor-pause.test.ts`.

Fusion-Task-Id: FN-3968

packages/engine/src/__tests__/executor-pause.test.ts

@@ -350,14 +350,21 @@ describe("Agent Spawning", () => {
 
     // createFnAgent is called at least twice: once for parent, once for child
     expect(mockedCreateFnAgent.mock.calls.length).toBeGreaterThanOrEqual(2);
-    
+
+    const parentCall = mockedCreateFnAgent.mock.calls.find(
+      (call: any) => !call[0].systemPrompt?.includes("child agent spawned")
+    );
     // Find the child session call
     const childCall = mockedCreateFnAgent.mock.calls.find(
       (call: any) => call[0].systemPrompt?.includes("child agent spawned")
     );
+
+    expect(parentCall).toBeDefined();
     expect(childCall).toBeDefined();
     expect(childCall![0].tools).toBe("coding");
+    expect(childCall![0].cwd).toContain("/.worktrees/swift-falcon");
     expect(childCall![0].systemPrompt).toContain("FN-SPAWN");
+    expect(childCall![0].taskEnv).toEqual(parentCall![0].taskEnv);
   });
 
   it("respects per-parent maxSpawnedAgentsPerParent limit", async () => {

packages/engine/src/__tests__/executor-step-session.test.ts

@@ -279,6 +279,7 @@ describe("Workflow Steps Execution", () => {
     expect(secondCall[0].tools).toBe("readonly");
     expect(secondCall[0].systemPrompt).toContain("Docs Review");
     expect(secondCall[0].systemPrompt).toContain("Review all docs and verify they are complete.");
+    expect(secondCall[0].taskEnv).toEqual(mockedCreateFnAgent.mock.calls[0][0].taskEnv);
 
     // Task should move to in-review
     expect(store.moveTask).toHaveBeenCalledWith("FN-001", "in-review");
@@ -816,6 +817,7 @@ describe("Workflow Steps Execution", () => {
 
   it("executes script-mode workflow step successfully", async () => {
     const store = createMockStore();
+    process.env.FN3968_SCRIPT_ENV = "workflow-script-env";
 
     store.getSettings.mockResolvedValue({
       maxConcurrent: 2,
@@ -906,6 +908,12 @@ describe("Workflow Steps Execution", () => {
     );
     const updatePayloads = store.updateTask.mock.calls.map((call: any[]) => call[1]);
     expect(JSON.stringify(updatePayloads)).not.toContain("all tests passed");
+
+    const scriptExecCall = mockedExecSync.mock.calls.find(
+      (call: any[]) => typeof call[0] === "string" && call[0].includes("echo 'all tests passed'")
+    );
+    expect(scriptExecCall?.[1]?.env?.FN3968_SCRIPT_ENV).toBe("workflow-script-env");
+    delete process.env.FN3968_SCRIPT_ENV;
   });
 
   it("executes plugin script-mode workflow step successfully", async () => {

packages/engine/src/__tests__/pi-create-fn-agent.test.ts

@@ -841,6 +841,7 @@ describe("createFnAgent", () => {
 
     const spawnHook = createBashToolMock.mock.calls.at(-1)?.[1]?.spawnHook;
     const originalEnv = { PATH: "/base/bin", HOME: "/home/user" };
+    const processEnvBefore = { ...process.env };
     const spawned = spawnHook({
       command: "echo hi",
       cwd: "/project",
@@ -857,6 +858,7 @@ describe("createFnAgent", () => {
       },
     });
     expect(originalEnv).toEqual({ PATH: "/base/bin", HOME: "/home/user" });
+    expect(process.env).toEqual(processEnvBefore);
   });
 
   it("keeps bash tool default behavior when taskEnv is not provided", async () => {
@@ -871,6 +873,71 @@ describe("createFnAgent", () => {
     expect(createBashToolMock).toHaveBeenCalledWith("/project", undefined);
   });
 
+  it("keeps spawned env unchanged when taskEnv is empty", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/project",
+      systemPrompt: "test",
+      tools: "coding",
+      taskEnv: {},
+    });
+
+    const spawnHook = createBashToolMock.mock.calls.at(-1)?.[1]?.spawnHook;
+    const originalEnv = { HOME: "/home/user", PATH: "/bin" };
+    const spawned = spawnHook({ command: "env", cwd: "/project", env: originalEnv });
+
+    expect(spawned.env).toEqual({ HOME: "/home/user", PATH: "/bin" });
+  });
+
+  it("adds new task env keys absent from spawned env", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/project",
+      systemPrompt: "test",
+      tools: "coding",
+      taskEnv: { TASK_ONLY: "abc" },
+    });
+
+    const spawnHook = createBashToolMock.mock.calls.at(-1)?.[1]?.spawnHook;
+    const spawned = spawnHook({ command: "env", cwd: "/project", env: { HOME: "/home/user" } });
+
+    expect(spawned.env).toEqual({ HOME: "/home/user", TASK_ONLY: "abc" });
+  });
+
+  it("preserves undefined task env values explicitly in merged env", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/project",
+      systemPrompt: "test",
+      tools: "coding",
+      taskEnv: { TASK_OPTIONAL: undefined },
+    });
+
+    const spawnHook = createBashToolMock.mock.calls.at(-1)?.[1]?.spawnHook;
+    const spawned = spawnHook({ command: "env", cwd: "/project", env: { HOME: "/home/user" } });
+
+    expect(spawned.env).toEqual({ HOME: "/home/user", TASK_OPTIONAL: undefined });
+  });
+
+  it("injects PATH from task env when spawned env has no PATH", async () => {
+    const { createFnAgent } = await import("../pi.js");
+
+    await createFnAgent({
+      cwd: "/project",
+      systemPrompt: "test",
+      tools: "coding",
+      taskEnv: { PATH: "/task/bin" },
+    });
+
+    const spawnHook = createBashToolMock.mock.calls.at(-1)?.[1]?.spawnHook;
+    const spawned = spawnHook({ command: "env", cwd: "/project", env: { HOME: "/home/user" } });
+
+    expect(spawned.env).toEqual({ HOME: "/home/user", PATH: "/task/bin" });
+  });
+
   it("refuses to start a coding agent in an unregistered worktree", async () => {
     existsSyncMock.mockImplementation((path) => {
       const value = String(path);