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/dotenv_load.go

@@ -0,0 +1,73 @@
+package infra
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// DotenvLoad parses a .env file and sets environment variables via os.Setenv.
+// It does NOT overwrite variables that are already set in the environment.
+//
+// Parsing rules:
+//   - Lines starting with # (after trimming) are ignored as comments.
+//   - Blank lines are ignored.
+//   - Lines must follow the KEY=VALUE format.
+//   - Values may be optionally quoted with single or double quotes; quotes are stripped.
+//   - Inline comments (# after value) are NOT stripped — the raw value after = is used.
+//
+// Returns an error if the file cannot be opened or if a line has no '=' separator.
+func DotenvLoad(path string) error {
+	f, err := os.Open(path)
+	if err != nil {
+		return fmt.Errorf("dotenv_load: open %q: %w", path, err)
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	lineNum := 0
+	for scanner.Scan() {
+		lineNum++
+		line := strings.TrimSpace(scanner.Text())
+
+		// skip blank lines and comments
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+
+		idx := strings.IndexByte(line, '=')
+		if idx < 0 {
+			return fmt.Errorf("dotenv_load: %q line %d: missing '=' in %q", path, lineNum, line)
+		}
+
+		key := strings.TrimSpace(line[:idx])
+		val := line[idx+1:]
+
+		// strip surrounding quotes from value
+		val = stripQuotes(val)
+
+		// only set if not already present
+		if os.Getenv(key) == "" {
+			if err := os.Setenv(key, val); err != nil {
+				return fmt.Errorf("dotenv_load: setenv %q: %w", key, err)
+			}
+		}
+	}
+
+	if err := scanner.Err(); err != nil {
+		return fmt.Errorf("dotenv_load: scan %q: %w", path, err)
+	}
+	return nil
+}
+
+// stripQuotes removes surrounding single or double quotes from a string.
+func stripQuotes(s string) string {
+	if len(s) >= 2 {
+		if (s[0] == '"' && s[len(s)-1] == '"') ||
+			(s[0] == '\'' && s[len(s)-1] == '\'') {
+			return s[1 : len(s)-1]
+		}
+	}
+	return s
+}