feat(process): capture stdout/stderr and fan out to subscribers

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

internal/process/subscription.go

@@ -0,0 +1,66 @@
+package process
+
+import "sync"
+
+// Subscription is a consumer handle for a process output stream.
+type Subscription interface {
+	Output() <-chan []byte
+	Close() error
+}
+
+// subscription is a buffered, closable fan-out channel for a Session.
+type subscription struct {
+	session *Session
+	ch      chan []byte
+	once    sync.Once
+	mu      sync.Mutex
+	closed  bool
+}
+
+func newSubscription(sess *Session) *subscription {
+	return &subscription{
+		session: sess,
+		ch:      make(chan []byte, 32),
+	}
+}
+
+func (s *subscription) Output() <-chan []byte {
+	return s.ch
+}
+
+func (s *subscription) Close() error {
+	s.once.Do(func() {
+		s.mu.Lock()
+		s.closed = true
+		close(s.ch)
+		s.mu.Unlock()
+		s.session.removeSubscription(s)
+	})
+	return nil
+}
+
+// closeChan closes the underlying channel exactly once without removing the
+// subscription from the session's subscriber list. It is used by the manager
+// when the process exits.
+func (s *subscription) closeChan() {
+	s.once.Do(func() {
+		s.mu.Lock()
+		s.closed = true
+		close(s.ch)
+		s.mu.Unlock()
+	})
+}
+
+// send performs a non-blocking send to the subscription channel. If the
+// channel is full or already closed, the chunk is dropped.
+func (s *subscription) send(chunk []byte) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.closed {
+		return
+	}
+	select {
+	case s.ch <- chunk:
+	default:
+	}
+}