feat: funciones impuras migration_create, migration_up, migration_down, migration_status

Fase 2 del issue 0015. MigrationCreate (crea archivo .sql template con version
auto-calculada), MigrationUp (aplica migraciones pendientes en transacciones
individuales), MigrationDown (revierte ultimas N via down_sql de _migrations),
MigrationGetStatus (cruza disco con BD, detecta orphaned). Tests de integracion:
ciclo completo create->up->status->down->status. 26 tests, todos pasan.

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

functions/infra/migration_create.go

@@ -0,0 +1,83 @@
+package infra
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+var migrationFilePattern = regexp.MustCompile(`^(\d+)_[a-zA-Z0-9_]+\.sql$`)
+var migrationNamePattern = regexp.MustCompile(`^[a-zA-Z][a-zA-Z0-9_]*$`)
+
+// MigrationCreate creates a new migration file in dir with the given name.
+// It calculates the next version by scanning existing .sql files in dir.
+// The filename follows the pattern NNN_name.sql (e.g. 003_add_index.sql).
+// Returns the absolute path of the created file.
+func MigrationCreate(dir, name string) (string, error) {
+	if !migrationNamePattern.MatchString(name) {
+		return "", fmt.Errorf("migration_create: name %q must match [a-zA-Z][a-zA-Z0-9_]*", name)
+	}
+
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return "", fmt.Errorf("migration_create: cannot create directory %q: %w", dir, err)
+	}
+
+	next, err := nextMigrationVersion(dir)
+	if err != nil {
+		return "", fmt.Errorf("migration_create: %w", err)
+	}
+
+	filename := fmt.Sprintf("%03d_%s.sql", next, name)
+	path := filepath.Join(dir, filename)
+
+	template := fmt.Sprintf("-- %s\n\n-- +up\n\n\n-- +down\n\n", filename)
+
+	if err := os.WriteFile(path, []byte(template), 0o644); err != nil {
+		return "", fmt.Errorf("migration_create: cannot write file %q: %w", path, err)
+	}
+
+	return path, nil
+}
+
+// nextMigrationVersion returns the next version number by scanning .sql files in dir.
+// Returns 1 if the directory is empty or has no migration files.
+func nextMigrationVersion(dir string) (int, error) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return 1, nil
+		}
+		return 0, fmt.Errorf("cannot read directory: %w", err)
+	}
+
+	max := 0
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		name := e.Name()
+		if !strings.HasSuffix(strings.ToLower(name), ".sql") {
+			continue
+		}
+		if !migrationFilePattern.MatchString(name) {
+			continue
+		}
+
+		idx := strings.Index(name, "_")
+		if idx < 0 {
+			continue
+		}
+		v, err := strconv.Atoi(name[:idx])
+		if err != nil {
+			continue
+		}
+		if v > max {
+			max = v
+		}
+	}
+
+	return max + 1, nil
+}