feat(browser): add timeout, idle auto-close, and max-pages safety mechanisms

Prevent resource leaks and hanging actions in the browser tool:
- Per-action context timeout (default 30s, configurable via timeoutMs param or config)
- Idle page reaper goroutine closes pages unused for 10min (configurable)
- Max pages per tenant (default 5) with LRU eviction
- RefStore cleanup on page close/evict/reap to prevent memory leaks

pkg/browser/refs.go

@@ -81,6 +81,14 @@ func NormalizeRef(raw string) string {
 	return s
 }
 
+// Remove deletes all refs for a target.
+func (rs *RefStore) Remove(targetID string) {
+	rs.mu.Lock()
+	defer rs.mu.Unlock()
+	rs.removeFromOrder(targetID)
+	delete(rs.entries, targetID)
+}
+
 func (rs *RefStore) removeFromOrder(targetID string) {
 	for i, id := range rs.order {
 		if id == targetID {