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/history.go

@@ -7,27 +7,51 @@ import (
 
 // Message is one entry in the conversation history.
 type Message struct {
-	Role string    `json:"role"`
-	Text string    `json:"text"`
-	Time time.Time `json:"time"`
+	Role      string    `json:"role"`
+	Text      string    `json:"text"`
+	Time      time.Time `json:"time"`
+	MessageID string    `json:"messageId,omitempty"`
 }
 
 // History accumulates prompt/response messages in memory for a session.
 type History struct {
-	mu       sync.RWMutex
-	messages []Message
+	mu        sync.RWMutex
+	messages  []Message
+	indexByID map[string]int
 }
 
 // NewHistory creates an empty History.
 func NewHistory() *History {
-	return &History{}
+	return &History{indexByID: make(map[string]int)}
 }
 
-// Add appends a message with the current time.
+// Add appends a message with the current time. It is used for messages that
+// have no messageId (e.g. the user prompt path).
 func (h *History) Add(role, text string) {
+	h.AppendOrCreate(role, "", text)
+}
+
+// AppendOrCreate finds the existing entry with the same messageId and appends
+// text to it; otherwise creates a new entry. When messageId is empty this
+// behaves like Add(role, text). It returns the time of the created or existing
+// entry.
+func (h *History) AppendOrCreate(role, messageId, text string) time.Time {
 	h.mu.Lock()
 	defer h.mu.Unlock()
-	h.messages = append(h.messages, Message{Role: role, Text: text, Time: time.Now()})
+
+	if messageId == "" {
+		h.messages = append(h.messages, Message{Role: role, Text: text, Time: time.Now()})
+		return h.messages[len(h.messages)-1].Time
+	}
+
+	if idx, ok := h.indexByID[messageId]; ok {
+		h.messages[idx].Text += text
+		return h.messages[idx].Time
+	}
+
+	h.messages = append(h.messages, Message{Role: role, Text: text, Time: time.Now(), MessageID: messageId})
+	h.indexByID[messageId] = len(h.messages) - 1
+	return h.messages[len(h.messages)-1].Time
 }
 
 // List returns a copy of all messages, oldest first.
@@ -66,4 +90,5 @@ func (h *History) Clear() {
 	h.mu.Lock()
 	defer h.mu.Unlock()
 	h.messages = h.messages[:0]
+	h.indexByID = make(map[string]int)
 }