chore: drop unused eslint dependency and empty gitkeep

`eslint` was only referenced by JSDoc type imports in the old `.mjs`
custom rules; the TypeScript migration dropped those, so the package
is no longer needed at runtime or for type-checking. The `scripts/`
directory now contains `postinstall.sh`, so the placeholder file is
also removed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: devmgn

package.json

@@ -70,7 +70,6 @@
     "@vitest/coverage-v8": "4.1.5",
     "axe-core": "4.11.3",
     "babel-plugin-react-compiler": "1.0.0",
-    "eslint": "10.2.1",
     "eslint-plugin-react-hooks": "7.1.1",
     "eslint-plugin-storybook": "10.3.5",
     "happy-dom": "20.9.0",