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/model/acp.go

@@ -37,3 +37,18 @@ type AcpPromptResponse struct {
 	StopReason string `json:"stopReason"`
 	Text       string `json:"text"`
 }
+
+// AcpStreamRequest is the first client message on the ACP streaming WebSocket.
+type AcpStreamRequest struct {
+	Type    string `json:"type"`
+	Content string `json:"content"`
+}
+
+// AcpStreamEvent is a server-to-client streaming event.
+type AcpStreamEvent struct {
+	Type       string `json:"type"`
+	MessageID  string `json:"messageId,omitempty"`
+	Text       string `json:"text,omitempty"`
+	StopReason string `json:"stopReason,omitempty"`
+	Error      string `json:"error,omitempty"`
+}