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

cmd/server/main.go

@@ -34,15 +34,16 @@ func main() {
 	}
 
 	workspaces := workspace.NewLocalManager(cfg.Workspace.Root)
-	processes := process.NewManager(cfg.Process.OpenCodeCommand)
+	processes := process.NewManager(cfg.Process.OpenCodeCommand, cfg.Process.Args)
 	shellMgr := shell.NewManager(cfg.Shell.Command, cfg.Shell.Args)
 
 	workspaceSvc := service.NewWorkspaceService(workspaces, processes, shellMgr, lg)
 	fileSvc := service.NewFileService(workspaces, cfg.File.MaxWriteBytes)
 	processSvc := service.NewProcessService(workspaces, processes, lg)
 	shellSvc := service.NewShellService(workspaces, shellMgr, lg)
+	acpSvc := service.NewAcpService(processes, workspaces, lg)
 
-	router := api.NewRouter(workspaceSvc, fileSvc, processSvc, shellSvc, lg, cfg.Gin.Mode)
+	router := api.NewRouter(workspaceSvc, fileSvc, processSvc, shellSvc, acpSvc, lg, cfg.Gin.Mode)
 
 	server := &http.Server{
 		Addr:           cfg.Server.Addr,