Adopt shared focused-contribution policy in data #82

Expose the published ecosystem CONTRIBUTING.md through a local contribution guide and focused issue/PR templates. Require one coherent problem per issue/PR, public-workflow bug reproduction and failing-base/passing-fix evidence, AI disclosure and contributor accountability. Preserve administrative and automated-dependency exceptions and the owning library compatibility requirements. No runtime behavior changes.

AI assistance: Codex prepared the changes and verification; maintainer review remains required.