feat(backend): add opencode ACP (Agent Client Protocol) client

Adds a minimal but real ACP stack for the opencode process:

- pkg/config: process.args default ["acp"] (opencodeCommand still "opencode")
- internal/process: NewManager(command, args) — exec.Command uses args
- internal/acp (new): NDJSON transport + JSON-RPC client over the existing
  process stdio. Implements initialize / session/new / session/prompt /
  session/cancel. Serves fs/read_text_file and fs/write_text_file from the
  workspace's fs.FileSystem. terminal/* requests get MethodNotFound.
- internal/service/acp_service: per-workspace Client + mutex; starts the
  process on first prompt; transparently re-init on restart.
- internal/api/acp_handler: GET /acp/status, GET /acp/history,
  POST /acp/prompt, POST /acp/cancel.
- internal/model/acp: API DTOs.
- internal/acp/client_test: NDJSON split-lines, request/response correlation,
  notification dispatch, agent-initiated request handling (fs + terminal).

Existing process WS endpoint and Shell subsystem are unchanged.

Conversation: 019f354c-a51b-7ec3-83ad-c647e9b50b19

internal/acp/history.go

@@ -0,0 +1,69 @@
+package acp
+
+import (
+	"sync"
+	"time"
+)
+
+// Message is one entry in the conversation history.
+type Message struct {
+	Role string    `json:"role"`
+	Text string    `json:"text"`
+	Time time.Time `json:"time"`
+}
+
+// History accumulates prompt/response messages in memory for a session.
+type History struct {
+	mu       sync.RWMutex
+	messages []Message
+}
+
+// NewHistory creates an empty History.
+func NewHistory() *History {
+	return &History{}
+}
+
+// Add appends a message with the current time.
+func (h *History) Add(role, text string) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	h.messages = append(h.messages, Message{Role: role, Text: text, Time: time.Now()})
+}
+
+// List returns a copy of all messages, oldest first.
+func (h *History) List() []Message {
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	out := make([]Message, len(h.messages))
+	copy(out, h.messages)
+	return out
+}
+
+// Len returns the number of stored messages.
+func (h *History) Len() int {
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	return len(h.messages)
+}
+
+// Since returns messages starting at index idx.
+func (h *History) Since(idx int) []Message {
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	if idx < 0 {
+		idx = 0
+	}
+	if idx > len(h.messages) {
+		idx = len(h.messages)
+	}
+	out := make([]Message, len(h.messages)-idx)
+	copy(out, h.messages[idx:])
+	return out
+}
+
+// Clear removes all stored messages.
+func (h *History) Clear() {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	h.messages = h.messages[:0]
+}