fix: ship scripts/uninstall.js in the npm package (#538)

The README tells users to run 'node scripts/uninstall.js' to clean up state
left outside the plugin folder, but the package files list didn't include
scripts/, so npm-installed users didn't have the file. Add scripts/uninstall.js
to files (its only dependency, hooks/, already ships). Guard it with a test
that asserts the files entry and that the file exists.

Co-authored-by: Anmolnoor <52135717+Anmolnoor@users.noreply.github.com>