feat: add Agnes AI as default provider, support multi-provider switching

- Default provider changed from Zhipu to Agnes AI (agnes-2.0-flash)
- Add --provider flag: agnes (default), zhipu, custom
- Add AGNES_API_KEY env var support (still compatible with ZHIPU_API_KEY)
- Add provider presets system for easy switching between AI backends
- Add .gitignore to prevent config.json with API keys from being committed
- Update usage examples

Co-Authored-By: Claude <noreply@anthropic.com>

main.go

@@ -18,12 +18,13 @@ func main() {
 		saveConfig bool
 	)
 
-	flag.StringVar(&apiKey, "apikey", "", "Zhipu AI API key (or set ZHIPU_API_KEY env var)")
+	flag.StringVar(&apiKey, "apikey", "", "API key (or set AGNES_API_KEY env var)")
 	flag.StringVar(&configPath, "config", "", "Path to config.json file")
 	flag.StringVar(&prompt, "prompt", "", "Custom prompt for image analysis")
 	flag.BoolVar(&quiet, "q", false, "Quiet mode — only output the model's response text")
 	flag.BoolVar(&noThink, "no-think", false, "Disable thinking mode (faster, lower server load)")
 	flag.BoolVar(&saveConfig, "save-config", false, "Save the API key to config.json and exit")
+	provider := flag.String("provider", "agnes", "AI provider: agnes (default), zhipu, or custom")
 	flag.Parse()
 
 	// Handle --save-config: persist the key and exit
@@ -50,8 +51,12 @@ func main() {
 		os.Exit(1)
 	}
 
+	// Apply provider preset (respects user's custom model/baseURL in config)
+	cfg.ApplyProvider(*provider)
+
 	if !quiet {
-		fmt.Fprintf(os.Stderr, "Using model: %s | API key: %s\n", cfg.Model, MaskKey(cfg.APIKey))
+		fmt.Fprintf(os.Stderr, "Provider: %s | Model: %s | API key: %s\n",
+			cfg.Provider, cfg.Model, MaskKey(cfg.APIKey))
 	}
 
 	// Collect image paths from remaining args
@@ -64,8 +69,9 @@ func main() {
 		fmt.Fprintln(os.Stderr, "")
 		fmt.Fprintln(os.Stderr, "Examples:")
 		fmt.Fprintln(os.Stderr, "  vision-tool photo.jpg")
-		fmt.Fprintln(os.Stderr, "  vision-tool -prompt \"这张图里有什么文字？\" screenshot.png")
+		fmt.Fprintln(os.Stderr, `  vision-tool -prompt "这张图里有什么文字？" screenshot.png`)
 		fmt.Fprintln(os.Stderr, "  vision-tool -q img1.png img2.png")
+		fmt.Fprintln(os.Stderr, "  vision-tool --provider zhipu photo.jpg")
 		fmt.Fprintln(os.Stderr, "  vision-tool --apikey YOUR_KEY photo.jpg")
 		fmt.Fprintln(os.Stderr, "  vision-tool --save-config --apikey YOUR_KEY")
 		os.Exit(1)