citizen: add contributing guide

CONTRIBUTING.md

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+# Contributing to sourcekeeper_42-contract-lab
+
+## Development Workflow
+
+### Prerequisites
+- [Foundry](https://book.getfoundry.sh/getting-started/installation) (forge, cast, anvil)
+- Node.js 18+ (for OpenZeppelin dependencies)
+- Git
+
+### Setup
+```bash
+git clone <repo-url>
+cd <repo-name>
+forge install
+npm install  # if using OpenZeppelin
+```
+
+### Build
+```bash
+forge build
+```
+
+### Test
+```bash
+forge test           # run all tests
+forge test -vvv      # verbose output
+forge test --gas-report  # gas report
+forge coverage       # coverage report
+```
+
+### Format
+```bash
+forge fmt
+```
+
+## Branch Naming
+
+Use conventional branch names:
+- `feat/<scope>-<description>` — new features
+- `fix/<scope>-<description>` — bug fixes
+- `chore/<scope>-<description>` — maintenance
+- `docs/<description>` — documentation updates
+- `test/<description>` — test additions/fixes
+
+## Commit Messages
+
+Follow [Conventional Commits](https://www.conventionalcommits.org/):
+```
+feat: add staking mechanism
+fix: correct overflow in reward calculation
+test: add fuzz tests for transfer
+docs: update deployment instructions
+```
+
+## Pull Requests
+
+1. Create a feature branch from `main`
+2. Write tests for new functionality
+3. Ensure all tests pass: `forge test`
+4. Ensure formatting: `forge fmt --check`
+5. Open PR with clear description of changes
+6. Wait for review approval
+
+## Security
+
+If you discover a security vulnerability, please report it privately.
+Do NOT open a public issue for security bugs.
+
+_Generated by Chunk Citizen._