Raise min heartbeat staleness floor 5m -> 10m

Agents stop heartbeating during long legitimate work (e.g. a verification
step blocked on a multi-minute test command). The 5-minute floor could
misread a busy agent as dead and reclaim its in-progress task mid-run.
Raise MIN_HEARTBEAT_STALENESS_MS to 10 minutes and strengthen the floor
test (7-minute-silent fast-interval agent stays healthy — would have read
stale under the old 5-minute floor). Engine typecheck clean; heartbeat
suite 148/148 pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.changeset/heartbeat-staleness-10min.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Raise the minimum agent heartbeat staleness floor from 5 to 10 minutes. Agents go silent during long-running but legitimate work (notably a verification step running a multi-minute test command, where the agent is blocked awaiting the command and cannot tick/heartbeat). The 5-minute floor could misread such a busy agent as dead and reclaim its in-progress task mid-run; 10 minutes gives long operations room before the liveness gate acts.

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

@@ -460,15 +460,18 @@ describe("executeHeartbeat", () => {
       expect(section).toMatch(/\| Long Interval \| active \| FN-209 \| .* \| healthy \|/);
     });
 
-    it("buildReportsHealthSection enforces a 5-minute minimum staleness floor", async () => {
+    it("buildReportsHealthSection enforces a 10-minute minimum staleness floor", async () => {
       const now = Date.now();
       const store = createStoreWithAgentForExec();
       vi.mocked(store.getCachedAgent).mockImplementation((id: string) => ({
         id,
         runtimeConfig: { heartbeatIntervalMs: 1_000 },
       }) as unknown as Agent);
+      // 7 minutes silent with a 1s interval: under the old 5-minute floor this
+      // would read as stale, but the 10-minute floor must still treat a busy
+      // long-running agent as healthy (e.g. mid multi-minute test command).
       vi.mocked(store.getAgentsByReportsTo).mockResolvedValue([
-        { id: "agent-fast", name: "Fast Poller", state: "active", taskId: "FN-210", lastHeartbeatAt: new Date(now - 2 * 60_000).toISOString(), updatedAt: new Date(now - 2 * 60_000).toISOString() } as Agent,
+        { id: "agent-fast", name: "Fast Poller", state: "active", taskId: "FN-210", lastHeartbeatAt: new Date(now - 7 * 60_000).toISOString(), updatedAt: new Date(now - 7 * 60_000).toISOString() } as Agent,
       ]);
       const monitor = new HeartbeatMonitor({ store, taskStore: mockTaskStore, rootDir: "/tmp" });
 

packages/engine/src/agent-heartbeat.ts

@@ -204,8 +204,11 @@ const REPORTS_STALE_INTERVAL_MULTIPLIER = 1.5;
 
 /**
  * Minimum staleness threshold floor for very short heartbeat intervals.
+ * 10 minutes: long-running but legitimately-busy agents (e.g. a verification
+ * step running a multi-minute test command, during which the agent does not
+ * tick/heartbeat) must not be misread as dead and reclaimed mid-run.
  */
-const MIN_HEARTBEAT_STALENESS_MS = 5 * 60_000;
+const MIN_HEARTBEAT_STALENESS_MS = 10 * 60_000;
 
 /** Format milliseconds into a human-readable duration string (e.g. "5m", "1h 20m", "2h"). */
 export function formatDuration(ms: number): string {