Create CONTRIBUTING.md/AGENTS.md

[BigLep]

Done Criteria

We have a CONTRIBUTING.md and AGENTS.md that makes clear our expectations for contributors.

Why Important

1. Contributors can know what's expected of them.
2. We can help agents help us to get what we want.
3. Review maintainer review time.

User/Customer

1. Contributors that want to "hack on" or contribute to this project.
2. Maintainers so they are more likely to get PRs that they're expecting.

Notes

1. For example we already see @timfong888 in https://github.com/timfong888/filecoin-pay-console wanting to explore things. He's using AI agents to help him (good call - makes sense). Lets set folks like him up for more success to potentially generate/author code that has a higher chance of being accepted.
2. I don't know the latest best practices here, but I would assume most of the content will go into CONTRIBUTING.md and AGENTS.md will will say "follow everything in CONTRIBUTING.md".

[timfong888]

Thanks,Steve. Related: will put a version in this repo of https://github.com/timfong888/filecoin-pay-console/blob/main/METRIC-DEFINITIONS.md

[pyropy]

Claude Code: Best practices for agentic coding by Anthropic could be used as guidelines.