fix: add package-lock.json for cache key + cache boolean input

[mayurrawte]

Root cause

actions/setup-node@v4 with cache: npm (or yarn/pnpm) requires a lock file to build the cache key. We deleted package.json/package-lock.json in v2, so all test jobs failed with:

Dependencies lock file is not found. Supported file patterns: package-lock.json,npm-shrinkwrap.json,yarn.lock

Fix

• Added minimal package.json + generated package-lock.json back to the repo (no dependencies, just metadata)
• Added cache boolean input to action.yml (default: true) — lets users disable caching when they have no lock file
• Cache is now conditionally passed to actions/setup-node: cache == 'true' && package-manager || ''
• Updated README inputs table