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/router_test.go

@@ -24,13 +24,14 @@ func setupTestRouterWithMaxWriteBytes(t *testing.T, maxWriteBytes int64) http.Ha
 	t.Helper()
 	root := t.TempDir()
 	wsMgr := workspace.NewLocalManager(root)
-	procMgr := process.NewManager("")
+	procMgr := process.NewManager("", nil)
 	shellMgr := shell.NewManager("bash", []string{"-i"})
 	wsSvc := service.NewWorkspaceService(wsMgr, procMgr, shellMgr, nil)
 	fileSvc := service.NewFileService(wsMgr, maxWriteBytes)
 	procSvc := service.NewProcessService(wsMgr, procMgr, nil)
 	shellSvc := service.NewShellService(wsMgr, shellMgr, nil)
-	return NewRouter(wsSvc, fileSvc, procSvc, shellSvc, nil, gin.TestMode)
+	acpSvc := service.NewAcpService(procMgr, wsMgr, nil)
+	return NewRouter(wsSvc, fileSvc, procSvc, shellSvc, acpSvc, nil, gin.TestMode)
 }
 
 func createWorkspaceForTest(t *testing.T, router http.Handler, id string) {