fix(FN-4257): align heartbeat cwd acquisition fallback

Fusion-Task-Id: FN-4257
Fusion-Task-Lineage: db48d51e-9e7d-471d-b26b-2a8763fb41d1

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

@@ -2211,7 +2211,7 @@ describe("executeHeartbeat", () => {
 
       expect(mockedCreateFnAgent).toHaveBeenCalledOnce();
       const callArgs = mockedCreateFnAgent.mock.calls[0]![0];
-      expect(callArgs.cwd).toBe("/tmp/test");
+      expect(callArgs.cwd).toBe("/tmp/worktree-fn-001");
       expect(callArgs.systemPrompt).toContain(HEARTBEAT_SYSTEM_PROMPT);
       expect(callArgs.systemPrompt).toContain("## Soul");
       expect(callArgs.systemPrompt).toContain("Act like a practical teammate who prioritizes clarity.");

packages/engine/src/agent-heartbeat.ts

@@ -2105,13 +2105,13 @@ export class HeartbeatMonitor {
         }
 
         let sessionCwd = rootDir;
-        if (!isNoTaskRun && taskDetail && heartbeatModelSettings) {
+        if (!isNoTaskRun && taskDetail) {
           try {
             const acquisition = await acquireTaskWorktree({
               task: taskDetail,
               rootDir,
               store: taskStore,
-              settings: heartbeatModelSettings,
+              settings: heartbeatModelSettings ?? {},
               logger: heartbeatLog,
               audit,
               runContext,