Add CONTRIBUTING.md

[christian-byrne]

What — No CONTRIBUTING.md, though DEV_README.md exists.

Why it matters here — 45 contributors on a public package, and GitHub's contribution prompts look for CONTRIBUTING.md specifically — a new contributor opening the PR form is pointed at nothing. DEV_README.md holds some of this content but is not in a place GitHub or a newcomer will find.

Evidence

$ ls CONTRIBUTING.md
ls: cannot access 'CONTRIBUTING.md': No such file or directory

$ ls DEV_README.md
DEV_README.md

Fix

Either rename DEV_README.md to CONTRIBUTING.md, or add a short CONTRIBUTING.md covering the required checks and PR conventions that links to it for the deeper setup.

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Found by repo-audit during repo improvement sweep 2026-08-17. Parent: #723