Add heartbeat event and daemon browser-liveness reaper

Introduce a `system.heartbeat` protocol event and use it as a
liveness signal on the daemon side.

- Track `last_seen` per connected browser, bumped on every inbound
  WS frame (tool response, event, ping/pong, or heartbeat).
- Add a reaper task that drops any browser silent past a 60s window
  and purges its sessions, mirroring the WS disconnect cleanup. This
  releases half-open connections left behind when the OS resumes from
  sleep and kills the service worker without delivering a close frame,
  so the daemon no longer pins a phantom "connected" browser and can
  idle-exit normally.
- Wire the reaper into both the foreground daemon and the test/embed
  entry point, aborting it on shutdown.

Co-authored-by: Cursor <cursoragent@cursor.com>

crates/bsk-cli/src/daemon/state.rs

@@ -82,6 +82,7 @@ pub struct DaemonHandle {
     ws: WsHandle,
     ipc: Option<IpcHandle>,
     session_idle_task: JoinHandle<()>,
+    browser_liveness_task: JoinHandle<()>,
 }
 
 impl DaemonHandle {
@@ -90,12 +91,14 @@ impl DaemonHandle {
         ws: WsHandle,
         ipc: Option<IpcHandle>,
         session_idle_task: JoinHandle<()>,
+        browser_liveness_task: JoinHandle<()>,
     ) -> Self {
         Self {
             state,
             ws,
             ipc,
             session_idle_task,
+            browser_liveness_task,
         }
     }
 
@@ -116,6 +119,8 @@ impl DaemonHandle {
     pub async fn shutdown(self) {
         self.session_idle_task.abort();
         let _ = await_join(self.session_idle_task).await;
+        self.browser_liveness_task.abort();
+        let _ = await_join(self.browser_liveness_task).await;
         self.ws.shutdown.notify_waiters();
         let _ = await_join(self.ws.task).await;
         if let Some(ipc) = self.ipc {