refactor(api)!: remove Delete method from public API

Environment variables are unaffected by Delete, making it misleading —
callers would expect a key to be fully gone but env vars would still
resolve. Sub is the better abstraction for scoped config.

setters.go

@@ -28,19 +28,6 @@ func (c *ConfigManager) Set(key string, value any) {
 	c.combinedConfig[lower] = ConfigMap{Key: key, Value: value}
 }
 
-// Delete removes a key from all configuration layers (overrides, file,
-// defaults) and rebuilds the combined configuration. Environment variables
-// are not affected since they are loaded from the process environment.
-func (c *ConfigManager) Delete(key string) {
-	c.mutex.Lock()
-	lower := strings.ToLower(key)
-	delete(c.overrideConfig, lower)
-	delete(c.fileConfig, lower)
-	delete(c.defaultConfig, lower)
-	delete(c.combinedConfig, lower)
-	c.mutex.Unlock()
-}
-
 func (c *ConfigManager) SetDefault(key string, value any) {
 	c.mutex.Lock()
 	defer c.mutex.Unlock()