feat(acp): streaming WebSocket route for prompt chunks

Adds GET /api/workspaces/:id/acp/stream. Client opens a WebSocket,
sends {"type":"prompt","content":"..."}, and receives a stream of
{"type":"chunk","messageId","text"} events followed by exactly
one {"type":"complete","stopReason"} or {"type":"error","error"}.
Closing the WS early triggers session/cancel.

- internal/acp/messages.go: StreamEvent wire shape.
- internal/acp/client.go:
  - streamChs []chan StreamEvent set; AddStream / RemoveStream.
  - sendStream non-blocking fanout.
  - Client.Stream(ctx, content, out) registers out, sends prompt,
    emits complete/error after the prompt response, unregisters.
  - handleNotification fans chunk events to all stream consumers.
  - notifyWG ensures chunk ordering vs the terminal event.
- internal/acp/service.go: Service.Stream(workspaceID, content, out)
  mirrors Prompt (per-workspace lock, 5-min timeout, EnsureReady).
- internal/service/acp_service.go: thin AcpService.Stream wrapper
  that maps acp.StreamEvent -> model.AcpStreamEvent.
- internal/model/acp.go: AcpStreamRequest, AcpStreamEvent DTOs.
- internal/api/acp_handler.go: stream WS handler (upgrade, read
  prompt, run Stream in a goroutine, write events, ping/pong, Cancel
  on client close).
- internal/api/router.go: register the new route.
- internal/acp/transport.go: dispatch notifications synchronously
  (vs. goroutine per notification) so chunks preserve order before
  the session/prompt response.
- internal/acp/client_test.go: TestClientStreamEmitsChunkAndComplete
  with a fake transport that drives a known sequence.
- internal/api/acp_handler_test.go: TestAcpStreamHandlerRoutes
  smoke test using a fake opencode acp script.

Existing POST /api/workspaces/:id/acp/prompt is unchanged.

E2E: prompt 'say hi in exactly 3 words' -> 3 chunk events
('Hi',' there','!') + 1 complete {stopReason: 'end_turn'}.

Conversation: 019f3680-200f-79b0-860b-43302e60d0ea

internal/service/acp_service.go

@@ -78,6 +78,35 @@ func (s *AcpService) Cancel(workspaceID string) error {
 	return nil
 }
 
+// Stream sends a prompt to the agent and forwards streamed events to out.
+func (s *AcpService) Stream(workspaceID string, content string, out chan<- model.AcpStreamEvent) error {
+	acpOut := make(chan acp.StreamEvent, 32)
+
+	var streamErr error
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		streamErr = s.svc.Stream(workspaceID, content, acpOut)
+		close(acpOut)
+	}()
+
+	for ev := range acpOut {
+		out <- model.AcpStreamEvent{
+			Type:       ev.Type,
+			MessageID:  ev.MessageID,
+			Text:       ev.Text,
+			StopReason: ev.StopReason,
+			Error:      ev.Error,
+		}
+	}
+	<-done
+
+	if streamErr != nil {
+		s.lg.Error("acp stream failed", "workspace_id", workspaceID, "error", streamErr)
+	}
+	return streamErr
+}
+
 // EnsureReady ensures the ACP process is started and initialized.
 func (s *AcpService) EnsureReady(workspaceID string) error {
 	if err := s.svc.EnsureReady(workspaceID); err != nil {