fix(acp): group agent chunks by messageId (backend) and coalesce consecutive same-role entries in the UI (frontend)

The opencode acp binary streams agent_message_chunk notifications with
an EMPTY messageId. The backend was already correct (it groups when a
messageId is present), but the UI rendered every chunk as its own
bubble, so a single 'count 1 to 5' reply showed as 9 separate bubbles.

Backend:
- internal/acp/history.go: add MessageID to Message; new AppendOrCreate
  method that finds or creates an entry by messageId and appends text
  to it (preserving the original timestamp).
- internal/acp/client.go: agent_message_chunk and user_message_chunk
  use AppendOrCreate.
- internal/model/acp.go: expose MessageID in the wire response.
- internal/service/acp_service.go: copy MessageID into the API DTO.
- internal/acp/client_test.go: add TestHistoryAppendOrCreateGroupsByMessageID.

Frontend:
- web/src/components/acp/groupMessages.ts: new pure helper that
  coalesces consecutive same-role history entries into a single bubble
  (keeps the first chunk's time + messageId).
- web/src/components/acp/AcpPanel.tsx: render coalesced messages;
  auto-scroll effect now depends on coalesced so it fires as the
  agent streams.

Verified via e2e: 'count from 1 to 5' now returns a single agent entry
with text '1\n2\n3\n4\n5' instead of 9 separate entries.

internal/acp/client.go

@@ -238,13 +238,13 @@ func (c *Client) handleNotification(method string, params json.RawMessage) {
 	switch up.Update.SessionUpdate {
 	case "agent_message_chunk":
 		if up.Update.Content.Type == "text" {
-			c.history.Add("agent", up.Update.Content.Text)
+			c.history.AppendOrCreate("agent", up.Update.MessageID, up.Update.Content.Text)
 		} else {
 			c.lg.Debug("acp drop non-text agent chunk", "workspace_id", c.workspaceID, "type", up.Update.Content.Type)
 		}
 	case "user_message_chunk":
 		if up.Update.Content.Type == "text" {
-			c.history.Add("user", up.Update.Content.Text)
+			c.history.AppendOrCreate("user", up.Update.MessageID, up.Update.Content.Text)
 		} else {
 			c.lg.Debug("acp drop non-text user chunk", "workspace_id", c.workspaceID, "type", up.Update.Content.Type)
 		}