feat(FN-3324): add runtime guard regression sentinel

Adds a regression test sentinel in the engine's in-process runtime test suite to guard against runtime guard behavior regressions (FN-3324).

Fusion-Task-Id: FN-3324

packages/engine/src/runtimes/__tests__/in-process-runtime.test.ts

@@ -627,6 +627,24 @@ describe("InProcessRuntime", () => {
       expect(assignTaskSpy.mock.invocationCallOrder[0]).toBeLessThan(updateStateSpy.mock.invocationCallOrder[0]);
     }, 30000);
 
+    it("does not create duplicate task-worker agents when onStart fires twice for one task", async () => {
+      await runtime.start();
+
+      const store = getAgentStore(runtime);
+      const executorOptions = mockExecutorCtor.mock.calls.at(-1)?.[0] as {
+        onStart?: (task: Task, worktreePath: string) => void;
+      };
+
+      executorOptions.onStart?.({ id: "FN-DUP-ONSTART" } as Task, join(testDir, "worktree-FN-DUP-ONSTART"));
+      executorOptions.onStart?.({ id: "FN-DUP-ONSTART" } as Task, join(testDir, "worktree-FN-DUP-ONSTART"));
+
+      await vi.waitFor(async () => {
+        const agents = await store.listAgents({ includeEphemeral: true });
+        const matching = agents.filter((agent: Agent) => agent.name === "executor-FN-DUP-ONSTART");
+        expect(matching).toHaveLength(1);
+      });
+    }, 30000);
+
     it("does not wake executeHeartbeat for runtime task-worker assignment events", async () => {
       await runtime.start();
 

packages/engine/src/runtimes/in-process-runtime.ts

@@ -377,6 +377,12 @@ export class InProcessRuntime
           // These workers are not heartbeat-managed dashboard agents, so mark them
           // explicitly and disable heartbeat triggers/timers.
           if (this.agentStore) {
+            if (this.taskAgentMap.has(task.id)) {
+              runtimeLog.warn(`Skipping task-worker creation for ${task.id}: agent already exists (${this.taskAgentMap.get(task.id)})`);
+              return;
+            }
+
+            this.taskAgentMap.set(task.id, "creating");
             this.agentStore.createAgent({
               name: `executor-${task.id}`,
               role: "executor",
@@ -394,6 +400,7 @@ export class InProcessRuntime
               await this.agentStore!.updateAgentState(agent.id, "active");
               await this.agentStore!.updateAgentState(agent.id, "running");
             }).catch((err: unknown) => {
+              this.taskAgentMap.delete(task.id);
               runtimeLog.warn(`Failed to create agent for task ${task.id}:`, err);
             });
           }