fix: improve type safety and error handling in config-manager

- Wrap JSON.parse in try-catch with clear error message for malformed JSON
- Add iflow, kiro, ghcp providers to CLIProxyVariantConfig type
- Make settings optional in CLIProxyVariantConfig (was required with empty string fallback)
- Remove unsafe type cast in loadConfigSafe()

Addresses additional edge cases from PR #215 code review.

src/types/config.ts

@@ -17,10 +17,10 @@ export interface ProfilesConfig {
  * Example: "flash" → gemini provider with gemini-2.5-flash model
  */
 export interface CLIProxyVariantConfig {
-  /** CLIProxy provider to use (gemini, codex, agy, qwen) */
-  provider: 'gemini' | 'codex' | 'agy' | 'qwen';
-  /** Path to settings.json with custom model configuration */
-  settings: string;
+  /** CLIProxy provider to use */
+  provider: 'gemini' | 'codex' | 'agy' | 'qwen' | 'iflow' | 'kiro' | 'ghcp';
+  /** Path to settings.json with custom model configuration (optional) */
+  settings?: string;
   /** Account identifier for multi-account support (optional, defaults to 'default') */
   account?: string;
   /** Unique port for variant isolation (8318-8417) */