Refactor and tidy code for release
- Factor out query parameter building with add_param/add_opt/add_int/add_bool helpers
- Simplify tag_ref conversion with tag_ref_of_poly helper
- Replace manual JSON printing with jsont codecs for proper encoding
- Use proper Eio error handling for JSON encoding errors
- Update dune-project with proper metadata (homepage, bug-reports)
- Clean up dune file formatting
- Add license headers to test files
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>