feat(FN-2328): add manager dropdown for agent reports-to setting

- Replace free-text Reports To input with a manager select populated from fetched agents
- Exclude the current agent from manager candidates and preserve unknown manager IDs as a fallback option
- Save selected manager IDs through updateAgent, including clearing reportsTo when No manager is selected
- Expand AgentDetailView settings tests for manager selection behavior and increase task document cascade test timeout stability

packages/core/src/__tests__/task-documents.test.ts

@@ -259,7 +259,7 @@ describe("TaskStore task documents", () => {
 
     const document = await store.getTaskDocument(task.id, "plan");
     expect(document).toBeNull();
-  });
+  }, 15_000);
 
   it("accepts valid key edge cases and rejects invalid ones", async () => {
     const task = await store.createTask({ description: "Key edge case task" });