feat: add IsSet, AllKeys, AllSettings; fix env precedence for file-only keys

- collapse() now applies env vars for keys present in fileConfig, not
  just defaultConfig. Previously, env vars couldn't override file values
  unless a default was also set for that key.
- SetDefault no longer pollutes overrideConfig; it correctly resolves
  the value by checking override > env > file > default.
- Remove unused explicitDefaults field and UseExplicitDefaults method.
- Add IsSet, AllKeys, AllSettings methods + package-level wrappers.
- Add missing package-level wrappers: Get, SetBool, SetString,
  SetConfigDir, WithEnvPrefix.
- Add tests for all new methods and the env-over-file-without-default
  fix.

default.go

@@ -67,3 +67,35 @@ func GetStringMap(key string) map[string]any {
 func GetStringSlice(key string) []string {
 	return defaultConfigManager.GetStringSlice(key)
 }
+
+func Get(key string) any {
+	return defaultConfigManager.Get(key)
+}
+
+func SetBool(key string, value bool) {
+	defaultConfigManager.SetBool(key, value)
+}
+
+func SetString(key string, value string) {
+	defaultConfigManager.SetString(key, value)
+}
+
+func SetConfigDir(path string) {
+	defaultConfigManager.SetConfigDir(path)
+}
+
+func WithEnvPrefix(prefix string) *ConfigManager {
+	return defaultConfigManager.WithEnvPrefix(prefix)
+}
+
+func IsSet(key string) bool {
+	return defaultConfigManager.IsSet(key)
+}
+
+func AllKeys() []string {
+	return defaultConfigManager.AllKeys()
+}
+
+func AllSettings() map[string]any {
+	return defaultConfigManager.AllSettings()
+}