feat(infra): funciones impuras Go para carga de env/config (dotenv, env_require, config_from_env, config_from_file)

- dotenv_load: parser .env con no-sobreescritura y soporte de comillas
- env_require: os.Getenv con error descriptivo fail-fast
- env_require_all: verifica multiples vars y lista todas las faltantes
- config_from_env: reflection sobre struct tags env/default/required/secret, 5 tipos soportados
- config_from_file: JSON via stdlib, YAML stub con not-implemented
- 25 tests unitarios, todos PASS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/config_from_file.go

@@ -0,0 +1,40 @@
+package infra
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// ConfigFromFile loads configuration from a file into target.
+// The target must be a non-nil pointer to a struct (same as json.Unmarshal).
+//
+// Supported formats:
+//   - JSON (.json) — full support via encoding/json
+//   - YAML (.yaml, .yml) — stub: returns "not implemented"
+//
+// The format is determined by the file extension (case-insensitive).
+func ConfigFromFile(path string, target any) error {
+	lower := strings.ToLower(path)
+
+	switch {
+	case strings.HasSuffix(lower, ".json"):
+		return loadJSON(path, target)
+	case strings.HasSuffix(lower, ".yaml"), strings.HasSuffix(lower, ".yml"):
+		return fmt.Errorf("config_from_file: YAML support not implemented; use a JSON file or add gopkg.in/yaml.v3")
+	default:
+		return fmt.Errorf("config_from_file: unsupported file extension for %q (supported: .json, .yaml, .yml)", path)
+	}
+}
+
+func loadJSON(path string, target any) error {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return fmt.Errorf("config_from_file: read %q: %w", path, err)
+	}
+	if err := json.Unmarshal(data, target); err != nil {
+		return fmt.Errorf("config_from_file: parse JSON %q: %w", path, err)
+	}
+	return nil
+}