fix(engine,core): dedup heartbeat-spawned follow-ups by parent task

Same-agent intake guard now also matches siblings sharing a
sourceParentTaskId, so repeated heartbeats from one parent task
can't bypass dedup just because triage rewrites the title.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/__tests__/agent-tools.test.ts

@@ -170,10 +170,40 @@ describe("createTaskCreateTool", () => {
     await tool.execute("call-1", { description: "Test" } as any, undefined, undefined, {} as any);
 
     expect(store.createTask).toHaveBeenCalledWith(expect.objectContaining({
-      source: { sourceType: "agent_heartbeat", sourceAgentId: "agent-123", sourceRunId: undefined },
+      source: {
+        sourceType: "agent_heartbeat",
+        sourceAgentId: "agent-123",
+        sourceRunId: undefined,
+        sourceParentTaskId: undefined,
+      },
     }), expect.objectContaining({ settings: { autoSummarizeTitles: false } }));
   });
 
+  it("threads sourceParentTaskId from provenance into store.createTask", async () => {
+    const store = {
+      getSettings: vi.fn().mockResolvedValue({ autoSummarizeTitles: false }),
+      createTask: vi.fn().mockResolvedValue({ id: "PROJ-101", description: "Test", dependencies: [], column: "triage" }),
+    };
+
+    const tool = createTaskCreateTool(store as any, {
+      sourceType: "agent_heartbeat",
+      sourceAgentId: "agent-123",
+      sourceRunId: "run-abc",
+      sourceParentTaskId: "FN-5206",
+    });
+
+    await tool.execute("call-1", { description: "spawn follow-up" } as any, undefined, undefined, {} as any);
+
+    expect(store.createTask).toHaveBeenCalledWith(expect.objectContaining({
+      source: {
+        sourceType: "agent_heartbeat",
+        sourceAgentId: "agent-123",
+        sourceRunId: "run-abc",
+        sourceParentTaskId: "FN-5206",
+      },
+    }), expect.anything());
+  });
+
   it("surfaces linked-existing text when deterministic duplicate is found", async () => {
     const existing = { id: "PROJ-500", description: "duplicate", dependencies: [], column: "triage" };
     vi.spyOn(core, "runDeterministicDuplicateGuard").mockResolvedValueOnce({