Add CONTRIBUTING.md for human (non-agent) first-time contributors

[cschanhniem]

Observation

The repo has excellent onboarding for AI agents — AGENTS.md, llms.txt, .cursorrules, .windsurfrules, CLAUDE.md — but a human landing on the repo for the first time has to piece together the contribution model from README.md, GOVERNANCE.md, QUICKSTART.md, and ROADMAP.md. A single CONTRIBUTING.md (separate from AGENTS.md) would lower the barrier for domain experts (public health researchers, policy analysts) who may want to review a submission, propose a problem pack, or flag a gap but aren`t comfortable navigating agent-oriented documentation.

Suggested addition

A CONTRIBUTING.md that answers, in order:

1. What can I contribute as a human? — Review submissions, propose new problem packs, improve datasets, file issues about evidence quality.
2. Where do I start? — Link to the problem pack that best matches their expertise, with a suggested first action (e.g., "read the evidence record and comment on the field-reality assessment").
3. How do reviews work? — Brief explanation of the review gates (schema validation → domain review → replication) in plain language.
4. What is the expected time commitment? — Even a ballpark ("a pack review takes 30-60 min for a domain familiar person") helps experts decide whether to engage.
5. How do I get help? — Link to discussions or a maintainer contact.

This does not duplicate AGENTS.md — that file is optimized for LLM consumption. CONTRIBUTING.md is optimized for a malaria epidemiologist who has never opened a GitHub Issue before.