fix(FN-2198): sync agent heartbeat config UI with live runtime updates

- Refresh AgentDetailView on agent:updated SSE events while preserving local unsaved config edits
- Resync heartbeat and budget form state from latest runtime config when agent data changes
- Centralize heartbeat interval defaults/formatting in shared utilities and reuse them in AgentsView selectors
- Add dashboard tests for default heartbeat hints, unset runtime fallback behavior, and custom interval options

packages/dashboard/app/components/__tests__/AgentDetailView.test.tsx

@@ -4,6 +4,7 @@ import userEvent from "@testing-library/user-event";
 import "@testing-library/jest-dom";
 import { AgentDetailView } from "../AgentDetailView";
 import type { AgentCapability, AgentDetail } from "../../api";
+import { DEFAULT_HEARTBEAT_INTERVAL_MS } from "../../utils/heartbeatIntervals";
 
 // Mock the API functions
 vi.mock("../../api", () => ({
@@ -1122,6 +1123,27 @@ describe("AgentDetailView", () => {
       });
     });
 
+    it("shows shared system default hint for heartbeat interval", async () => {
+      mockFetchAgent.mockResolvedValue(createMockAgent({ metadata: {} }));
+
+      const user = userEvent.setup();
+      render(
+        <AgentDetailView
+          agentId="agent-001"
+          onClose={vi.fn()}
+          addToast={vi.fn()}
+        />
+      );
+
+      await navigateToSettings(user);
+
+      const heartbeatInput = await screen.findByLabelText("Heartbeat Interval (ms)");
+      expect(heartbeatInput).toHaveAttribute("placeholder", String(DEFAULT_HEARTBEAT_INTERVAL_MS));
+      expect(
+        screen.getByText(`How often heartbeats are checked. Leave empty for system default (${DEFAULT_HEARTBEAT_INTERVAL_MS}ms / 1h).`),
+      ).toBeInTheDocument();
+    });
+
     it("pre-fills heartbeat fields from agent runtimeConfig", async () => {
       mockFetchAgent.mockResolvedValue(createMockAgent({
         runtimeConfig: {

packages/dashboard/app/components/__tests__/AgentsView.test.tsx

@@ -181,6 +181,25 @@ describe("AgentsView", () => {
       expect(screen.getByDisplayValue("30s")).toBeTruthy();
     });
 
+    it("uses the system default heartbeat interval when runtime config is unset", async () => {
+      mockFetchAgents.mockResolvedValue([
+        {
+          ...mockAgents[1],
+          runtimeConfig: {},
+        },
+      ]);
+
+      render(<AgentsView addToast={mockAddToast} />);
+
+      await waitFor(() => {
+        expect(screen.getByLabelText("Set heartbeat interval for Test Agent 2")).toBeTruthy();
+      });
+
+      const intervalSelect = screen.getByLabelText("Set heartbeat interval for Test Agent 2") as HTMLSelectElement;
+      expect(intervalSelect.value).toBe("3600000");
+      expect(intervalSelect.options[intervalSelect.selectedIndex]?.text).toBe("1h");
+    });
+
     it("updates agent heartbeat interval from preset dropdown", async () => {
       render(<AgentsView addToast={mockAddToast} />);
 
@@ -202,7 +221,7 @@ describe("AgentsView", () => {
       });
     });
 
-    it("maps non-preset heartbeat interval to closest preset", async () => {
+    it("shows a custom heartbeat option when configured interval is not a preset", async () => {
       mockFetchAgents.mockResolvedValue([
         {
           ...mockAgents[1],
@@ -217,8 +236,9 @@ describe("AgentsView", () => {
       });
 
       const intervalSelect = screen.getByLabelText("Set heartbeat interval for Test Agent 2") as HTMLSelectElement;
-      expect(intervalSelect.value).toBe("60000");
-      expect(screen.getAllByText("1m").length).toBeGreaterThan(0);
+      expect(intervalSelect.value).toBe("65000");
+      expect(intervalSelect.options[intervalSelect.selectedIndex]?.text).toBe("1m (custom)");
+      expect(screen.getByRole("option", { name: "1m (custom)" })).toBeTruthy();
     });
 
     it("shows refresh button", async () => {