feat(codex): 移除 opencode 指令回源与缓存

- 不再从 GitHub 拉取 opencode codex_header.txt\n- 删除 ~/.opencode 缓存与异步刷新逻辑\n- 所有 instructions 统一使用内置 codex_cli_instructions.md

backend/internal/service/openai_codex_transform_test.go

@@ -1,11 +1,7 @@
 package service
 
 import (
-	"encoding/json"
-	"os"
-	"path/filepath"
 	"testing"
-	"time"
 
 	"github.com/stretchr/testify/require"
 )
@@ -191,23 +187,6 @@ func TestNormalizeCodexModel_Gpt53(t *testing.T) {
 
 func setupCodexCache(t *testing.T) {
 	t.Helper()
-
-	// 使用临时 HOME 避免触发网络拉取 header。
-	tempDir := t.TempDir()
-	t.Setenv("HOME", tempDir)
-
-	cacheDir := filepath.Join(tempDir, ".opencode", "cache")
-	require.NoError(t, os.MkdirAll(cacheDir, 0o755))
-	require.NoError(t, os.WriteFile(filepath.Join(cacheDir, "opencode-codex-header.txt"), []byte("header"), 0o644))
-
-	meta := map[string]any{
-		"etag":        "",
-		"lastFetch":   time.Now().UTC().Format(time.RFC3339),
-		"lastChecked": time.Now().UnixMilli(),
-	}
-	data, err := json.Marshal(meta)
-	require.NoError(t, err)
-	require.NoError(t, os.WriteFile(filepath.Join(cacheDir, "opencode-codex-header-meta.json"), data, 0o644))
 }
 
 func TestApplyCodexOAuthTransform_CodexCLI_PreservesExistingInstructions(t *testing.T) {