Implement environment variable support for API keys

Co-authored-by: m7medVision <88824957+m7medVision@users.noreply.github.com>

Author: Copilot

internal/config/config.go

@@ -100,7 +100,19 @@ func GetAPIKey() (string, error) {
 		return "", fmt.Errorf("API key for provider '%s' is not set", cfg.ActiveProvider)
 	}
 
-	return providerConfig.APIKey, nil
+	apiKey := providerConfig.APIKey
+
+	// Check if the API key is an environment variable reference
+	if strings.HasPrefix(apiKey, "$") {
+		envVarName := strings.TrimPrefix(apiKey, "$")
+		envValue := os.Getenv(envVarName)
+		if envValue == "" {
+			return "", fmt.Errorf("environment variable '%s' for provider '%s' is not set or empty", envVarName, cfg.ActiveProvider)
+		}
+		return envValue, nil
+	}
+
+	return apiKey, nil
 }
 
 // GetModel returns the model for the active provider.

internal/config/config_test.go

@@ -0,0 +1,106 @@
+package config
+
+import (
+	"os"
+	"testing"
+
+	"github.com/spf13/viper"
+)
+
+func TestGetAPIKey_EnvironmentVariable(t *testing.T) {
+	// Reset configuration for clean test
+	cfg = nil
+	viper.Reset()
+
+	// Set up test environment variable
+	testEnvVar := "TEST_API_KEY"
+	testAPIKey := "test-api-key-value-123"
+	os.Setenv(testEnvVar, testAPIKey)
+	defer os.Unsetenv(testEnvVar)
+
+	// Initialize config
+	InitConfig()
+
+	// Set up test provider with environment variable reference
+	testProvider := "openrouter"
+	cfg.ActiveProvider = testProvider
+	if cfg.Providers == nil {
+		cfg.Providers = make(map[string]ProviderConfig)
+	}
+	cfg.Providers[testProvider] = ProviderConfig{
+		APIKey: "$" + testEnvVar,
+		Model:  "test-model",
+	}
+
+	// Test that environment variable is resolved
+	resolvedKey, err := GetAPIKey()
+	if err != nil {
+		t.Fatalf("Expected no error, got: %v", err)
+	}
+
+	if resolvedKey != testAPIKey {
+		t.Errorf("Expected resolved API key to be %s, got %s", testAPIKey, resolvedKey)
+	}
+}
+
+func TestGetAPIKey_EnvironmentVariableNotSet(t *testing.T) {
+	// Reset configuration for clean test
+	cfg = nil
+	viper.Reset()
+
+	// Initialize config
+	InitConfig()
+
+	// Set up test provider with environment variable reference that doesn't exist
+	testProvider := "openrouter"
+	cfg.ActiveProvider = testProvider
+	if cfg.Providers == nil {
+		cfg.Providers = make(map[string]ProviderConfig)
+	}
+	cfg.Providers[testProvider] = ProviderConfig{
+		APIKey: "$NONEXISTENT_API_KEY",
+		Model:  "test-model",
+	}
+
+	// Test that missing environment variable returns error
+	_, err := GetAPIKey()
+	if err == nil {
+		t.Fatal("Expected error for missing environment variable, got nil")
+	}
+
+	expectedError := "environment variable 'NONEXISTENT_API_KEY' for provider 'openrouter' is not set or empty"
+	if err.Error() != expectedError {
+		t.Errorf("Expected error message '%s', got '%s'", expectedError, err.Error())
+	}
+}
+
+func TestGetAPIKey_RegularAPIKey(t *testing.T) {
+	// Reset configuration for clean test
+	cfg = nil
+	viper.Reset()
+
+	// Initialize config
+	InitConfig()
+
+	// Set up test provider with regular API key (not environment variable)
+	testProvider := "openrouter"
+	testAPIKey := "regular-api-key-123"
+	cfg.ActiveProvider = testProvider
+	if cfg.Providers == nil {
+		cfg.Providers = make(map[string]ProviderConfig)
+	}
+	cfg.Providers[testProvider] = ProviderConfig{
+		APIKey: testAPIKey,
+		Model:  "test-model",
+	}
+
+	// Test that regular API key is returned as-is
+	resolvedKey, err := GetAPIKey()
+	if err != nil {
+		t.Fatalf("Expected no error, got: %v", err)
+	}
+
+	if resolvedKey != testAPIKey {
+		t.Errorf("Expected API key to be %s, got %s", testAPIKey, resolvedKey)
+	}
+}