fix(writers): drop version stamps from generated output

Generated files carried the RelSpec version and build date, so every
rebuild produced a diff even when the schema was unchanged.

- bun, gorm, drizzle: remove the GeneratedBy template field and line
- pgsql, mssql, sqlite: emit a constant "-- Generated by RelSpec"
- bun: replace the version header test with a version-free assertion

pkg/buildinfo is unchanged and still backs the CLI version.

pkg/writers/pgsql/migration_writer.go

@@ -7,7 +7,6 @@ import (
 	"sort"
 	"strings"
 
-	"git.warky.dev/wdevs/relspecgo/pkg/buildinfo"
 	"git.warky.dev/wdevs/relspecgo/pkg/models"
 	"git.warky.dev/wdevs/relspecgo/pkg/pgsql"
 	"git.warky.dev/wdevs/relspecgo/pkg/writers"
@@ -144,7 +143,7 @@ func (w *MigrationWriter) WriteMigration(model, current *models.Database) error
 
 	// Write header
 	fmt.Fprintf(w.writer, "-- PostgreSQL Migration Script\n")
-	fmt.Fprintf(w.writer, "-- Generated by %s\n", buildinfo.GeneratedComment())
+	fmt.Fprintf(w.writer, "-- Generated by RelSpec\n")
 	fmt.Fprintf(w.writer, "-- Source: %s -> %s\n", current.Name, model.Name)
 	if w.options.ContinueOnError {
 		fmt.Fprintf(w.writer, "\\set ON_ERROR_STOP off\n")