fix(mcp): correct Codex MCP configuration format to [mcp_servers]

BREAKING CHANGE: The [mcp.servers] format was completely incorrect and not
any official Codex format. The only correct format is [mcp_servers] at the
top level of config.toml.

Changes:
- Remove incorrect [mcp.servers] nested table support
- Always use [mcp_servers] top-level table (official Codex format)
- Auto-migrate and cleanup erroneous [mcp.servers] entries on write
- Preserve error-tolerant import for migrating old incorrect configs
- Simplify sync logic by removing format selection branches (~60 lines)
- Update all documentation and tests to reflect correct format
- Add warning logs when detecting and cleaning incorrect format

Backend (Rust):
- mcp.rs: Simplify sync_enabled_to_codex by removing Target enum
- mcp.rs: sync_single_server_to_codex now always uses [mcp_servers]
- mcp.rs: remove_server_from_codex cleans both locations
- mcp.rs: Update import_from_codex comments to clarify format status
- tests: Rename test to sync_enabled_to_codex_migrates_erroneous_*
- tests: Update assertions to verify migration behavior

Frontend (TypeScript):
- tomlUtils.ts: Prioritize [mcp_servers] format in parsing
- tomlUtils.ts: Update error messages to guide correct format

Documentation:
- README.md: Correct MCP format reference to [mcp_servers]
- CLAUDE.md: Add comprehensive format specification with examples

All 79 tests pass. This ensures backward compatibility while enforcing
the correct Codex official standard going forward.

Refs: https://github.com/openai/codex/issues/3441

src-tauri/src/app_config.rs

@@ -317,7 +317,9 @@ impl MultiAppConfig {
 
         // 迁移通用配置片段：claude_common_config_snippet → common_config_snippets.claude
         if let Some(old_claude_snippet) = config.claude_common_config_snippet.take() {
-            log::info!("迁移通用配置：claude_common_config_snippet → common_config_snippets.claude");
+            log::info!(
+                "迁移通用配置：claude_common_config_snippet → common_config_snippets.claude"
+            );
             config.common_config_snippets.claude = Some(old_claude_snippet);
             updated = true;
         }
@@ -414,9 +416,7 @@ impl MultiAppConfig {
             return Ok(false);
         }
 
-        log::info!(
-            "检测到已存在配置文件且 Prompt 列表为空，将尝试从现有提示词文件自动导入"
-        );
+        log::info!("检测到已存在配置文件且 Prompt 列表为空，将尝试从现有提示词文件自动导入");
 
         let mut imported = false;
         for app in [AppType::Claude, AppType::Codex, AppType::Gemini] {