[ TheCryptoDonkey/geohash-kit ] docs: improve AI discoverability

- llms.txt: lead with problem statement, not feature description
- package.json: add 7 missing keywords (ngeohash, geofencing, coordinates, etc.)
- CONTRIBUTING.md: add contributor guide with setup, conventions, and PR workflow
https://github.com/TheCryptoDonkey/geohash-kit/commit/1215b95f776cc03b54296f5f4972d25c65ccd15e