feat(config, usage): add usage-statistics-enabled option and dynamic toggling

- Introduced `usage-statistics-enabled` configuration to control in-memory usage aggregation.
- Updated API to include handlers for managing `usage-statistics-enabled` and `logging-to-file` options.
- Enhanced `watcher` to log changes to both configurations dynamically.
- Updated documentation and examples to reflect new configuration options.

internal/config/config.go

@@ -26,6 +26,9 @@ type Config struct {
 	// LoggingToFile controls whether application logs are written to rotating files or stdout.
 	LoggingToFile bool `yaml:"logging-to-file" json:"logging-to-file"`
 
+	// UsageStatisticsEnabled toggles in-memory usage aggregation; when false, usage data is discarded.
+	UsageStatisticsEnabled bool `yaml:"usage-statistics-enabled" json:"usage-statistics-enabled"`
+
 	// ProxyURL is the URL of an optional proxy server to use for outbound requests.
 	ProxyURL string `yaml:"proxy-url" json:"proxy-url"`
 
@@ -206,6 +209,7 @@ func LoadConfig(configFile string) (*Config, error) {
 	var config Config
 	// Set defaults before unmarshal so that absent keys keep defaults.
 	config.LoggingToFile = true
+	config.UsageStatisticsEnabled = true
 	config.GeminiWeb.Context = true
 	if err = yaml.Unmarshal(data, &config); err != nil {
 		return nil, fmt.Errorf("failed to parse config file: %w", err)