fix(codex): 添加codex CLI instructions fallback机制

## 问题
- 使用OpenAI API key时，opencode客户端可能因instructions不兼容而报错
- 依赖外部GitHub获取instructions，网络故障时会失败

## 解决方案
1. 将codex CLI标准instructions嵌入到项目中
2. 实现自动fallback机制：
   - 优先使用opencode GitHub的instructions
   - 失败时自动fallback到本地codex CLI instructions
3. 添加辅助函数用于错误检测和手动替换

## 改动
- 新增: internal/service/prompts/codex_cli_instructions.md
  - 从codex项目复制的标准instructions
  - 使用go:embed嵌入到二进制文件

- 修改: internal/service/openai_codex_transform.go
  - 添加embed支持
  - 增强getOpenCodeCodexHeader()的fallback逻辑
  - 新增GetCodexCLIInstructions()公开函数
  - 新增ReplaceWithCodexInstructions()用于手动替换
  - 新增IsInstructionError()用于错误检测

## 优势
- 零停机：GitHub不可用时仍能正常工作
- 离线可用：不依赖外部网络
- 兼容性：使用标准codex CLI instructions
- 部署简单：instructions嵌入到二进制文件

backend/internal/service/openai_codex_transform.go

@@ -1,6 +1,7 @@
 package service
 
 import (
+	_ "embed"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -16,6 +17,9 @@ const (
 	codexCacheTTL          = 15 * time.Minute
 )
 
+//go:embed prompts/codex_cli_instructions.md
+var codexCLIInstructions string
+
 var codexModelMap = map[string]string{
 	"gpt-5.1-codex":             "gpt-5.1-codex",
 	"gpt-5.1-codex-low":         "gpt-5.1-codex",
@@ -119,6 +123,13 @@ func applyCodexOAuthTransform(reqBody map[string]any) codexTransformResult {
 			reqBody["instructions"] = instructions
 			result.Modified = true
 		}
+	} else if existingInstructions == "" {
+		// If no opencode instructions available, try codex CLI instructions
+		codexInstructions := strings.TrimSpace(getCodexCLIInstructions())
+		if codexInstructions != "" {
+			reqBody["instructions"] = codexInstructions
+			result.Modified = true
+		}
 	}
 
 	if input, ok := reqBody["input"].([]any); ok {
@@ -235,13 +246,69 @@ func getOpenCodeCachedPrompt(url, cacheFileName, metaFileName string) string {
 }
 
 func getOpenCodeCodexHeader() string {
-	return getOpenCodeCachedPrompt(opencodeCodexHeaderURL, "opencode-codex-header.txt", "opencode-codex-header-meta.json")
+	// Try to get from opencode repository first
+	opencodeInstructions := getOpenCodeCachedPrompt(opencodeCodexHeaderURL, "opencode-codex-header.txt", "opencode-codex-header-meta.json")
+
+	// If opencode instructions are available, return them
+	if opencodeInstructions != "" {
+		return opencodeInstructions
+	}
+
+	// Fallback to local codex CLI instructions
+	return getCodexCLIInstructions()
+}
+
+func getCodexCLIInstructions() string {
+	return codexCLIInstructions
 }
 
 func GetOpenCodeInstructions() string {
 	return getOpenCodeCodexHeader()
 }
 
+func GetCodexCLIInstructions() string {
+	return getCodexCLIInstructions()
+}
+
+func ReplaceWithCodexInstructions(reqBody map[string]any) bool {
+	codexInstructions := strings.TrimSpace(getCodexCLIInstructions())
+	if codexInstructions == "" {
+		return false
+	}
+
+	existingInstructions, _ := reqBody["instructions"].(string)
+	if strings.TrimSpace(existingInstructions) != codexInstructions {
+		reqBody["instructions"] = codexInstructions
+		return true
+	}
+
+	return false
+}
+
+func IsInstructionError(errorMessage string) bool {
+	if errorMessage == "" {
+		return false
+	}
+
+	lowerMsg := strings.ToLower(errorMessage)
+	instructionKeywords := []string{
+		"instruction",
+		"instructions",
+		"system prompt",
+		"system message",
+		"invalid prompt",
+		"prompt format",
+	}
+
+	for _, keyword := range instructionKeywords {
+		if strings.Contains(lowerMsg, keyword) {
+			return true
+		}
+	}
+
+	return false
+}
+
 func filterCodexInput(input []any) []any {
 	filtered := make([]any, 0, len(input))
 	for _, item := range input {