Add CONTRIBUTING.md and CODE_OF_CONDUCT.md

[tchjhnsn]

Basic community infrastructure for an open-source project. Suggested templates:

• CONTRIBUTING.md: how to file issues, expected PR workflow, dev setup pointer, maintenance-cadence note ("solo, best effort")
• CODE_OF_CONDUCT.md: Contributor Covenant 2.1 is the standard choice

Also consider enabling GitHub Discussions if you want a community forum for research/architecture conversations separate from issue tracking.