Update .gitattributes with lean-package-validator

Use lean-package-validator to refresh an existing .gitattributes file:

1. Run ./vendor/bin/lean-package-validator update to replace missing export-ignore entries.
2. Add --align-export-ignores if you want aligned columns.
3. Add --omit-header to avoid modifying the header comment.
4. If you need a preview first, run ./vendor/bin/lean-package-validator validate --diff and then re-run with the update command.

Keep any non-export-ignore rules intact; lean-package-validator appends export-ignores below them.