refactor(process): use map for subscribers and drop dead ring buffer

Co-Authored-By: Claude <noreply@anthropic.com>

internal/process/session.go

@@ -25,7 +25,7 @@ type Session struct {
 	Root        string
 	Cmd         *exec.Cmd
 	Stdin       io.WriteCloser
-	Subscribers []Subscription
+	Subscribers map[Subscription]struct{}
 	Exit        ExitInfo
 	mu          sync.Mutex
 }
@@ -35,10 +35,5 @@ type Session struct {
 func (s *Session) removeSubscription(sub *subscription) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	for i, cur := range s.Subscribers {
-		if cur == sub {
-			s.Subscribers = append(s.Subscribers[:i], s.Subscribers[i+1:]...)
-			return
-		}
-	}
+	delete(s.Subscribers, sub)
 }