docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to ProofWorks
+
+Thanks for taking the time to look at this. The project is small enough that one good PR can move it forward visibly.
+
+## Setup
+
+Clone the repo and install everything from the project root:
+
+```bash
+pip install -r requirements.txt
+npm install
+npm --prefix frontend install
+```
+
+A working Python 3.11+ and Node 20+ environment is assumed.
+
+## Running the tests
+
+The contract suite uses `pytest` via `genlayer-test` direct mode and runs in about four seconds:
+
+```bash
+make test
+```
+
+You should see `89 passed`. If you see SDK loader errors, make sure `scripts/setup_genvmroot.py` ran successfully (the `make test` target invokes it for you).
+
+To lint the contract against the GenVM static checker:
+
+```bash
+make lint-contract
+```
+
+## Submitting a pull request
+
+1. Open or claim an issue first so we can agree on the scope.
+2. Branch from `main` and keep the PR focused on one logical change.
+3. Make sure `make test` and `make lint-contract` both pass locally.
+4. Reference the issue in the PR body with a closing keyword (`Closes #123`).
+5. Open the PR from a branch on this same repository so the contract's same-repo check passes.
+
+For larger changes, the `spec.md` (in the repository's earlier history) and `docs/architecture.md` give context on how the contract is structured.