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/model/acp.go

@@ -0,0 +1,38 @@
+package model
+
+import "time"
+
+// AcpStatusResponse is the API response for ACP status.
+type AcpStatusResponse struct {
+	WorkspaceID string `json:"workspaceId"`
+	Ready       bool   `json:"ready"`
+	SessionID   string `json:"sessionId,omitempty"`
+	Running     bool   `json:"running"`
+	PID         int    `json:"pid,omitempty"`
+	Error       string `json:"error,omitempty"`
+}
+
+// AcpMessage is a single role/text/time entry in the ACP history.
+type AcpMessage struct {
+	Role string    `json:"role"`
+	Text string    `json:"text"`
+	Time time.Time `json:"time"`
+}
+
+// AcpHistoryResponse is the API response for ACP history.
+type AcpHistoryResponse struct {
+	SessionID string       `json:"sessionId"`
+	Messages  []AcpMessage `json:"messages"`
+}
+
+// AcpPromptRequest is the body for POST /acp/prompt.
+type AcpPromptRequest struct {
+	Content string `json:"content"`
+}
+
+// AcpPromptResponse is the API response for a prompt turn.
+type AcpPromptResponse struct {
+	SessionID  string `json:"sessionId"`
+	StopReason string `json:"stopReason"`
+	Text       string `json:"text"`
+}