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/api/acp_handler.go

@@ -0,0 +1,69 @@
+package api
+
+import (
+	"net/http"
+
+	"codespace/internal/model"
+	"codespace/internal/service"
+
+	"github.com/gin-gonic/gin"
+)
+
+type acpHandler struct {
+	svc *service.AcpService
+}
+
+func (h *acpHandler) status(c *gin.Context) {
+	id := c.Param("id")
+	status, err := h.svc.Status(id)
+	if err != nil {
+		writeError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, model.AcpStatusResponse{
+		WorkspaceID: status.WorkspaceID,
+		Ready:       status.Ready,
+		SessionID:   status.SessionID,
+		Running:     status.Running,
+		PID:         status.PID,
+		Error:       status.Error,
+	})
+}
+
+func (h *acpHandler) history(c *gin.Context) {
+	id := c.Param("id")
+	hist, err := h.svc.History(id)
+	if err != nil {
+		writeError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, hist)
+}
+
+func (h *acpHandler) prompt(c *gin.Context) {
+	id := c.Param("id")
+	var req model.AcpPromptRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		writeBadRequest(c, "invalid json")
+		return
+	}
+	if req.Content == "" {
+		writeBadRequest(c, "content is required")
+		return
+	}
+	res, err := h.svc.Prompt(id, req.Content)
+	if err != nil {
+		writeError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, res)
+}
+
+func (h *acpHandler) cancel(c *gin.Context) {
+	id := c.Param("id")
+	if err := h.svc.Cancel(id); err != nil {
+		writeError(c, err)
+		return
+	}
+	c.Status(http.StatusNoContent)
+}