docs: Add CONTRIBUTING.md with build, test, mage usage, and PR guidelines

- Quick start guide for contributors
- Build instructions (mage and go direct)
- Test instructions (mage test, mage testRPN, coverage)
- Code style guidelines (Go best practices, documentation)
- PR guidelines (before submitting, description, review process)
- Development workflow (branching, committing, pushing)

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+# Contributing to gt
+
+Thank you for your interest in contributing to `gt`! This document provides guidelines for building, testing, and submitting contributions.
+
+## Quick Start
+
+1. Fork the repository
+2. Create a branch for your feature or fix
+3. Make your changes
+4. Run tests and build
+5. Submit a pull request
+
+## Building
+
+### Using Mage (Recommended)
+
+Install mage if you haven't already:
+```bash
+go install github.com/magefile/mage@latest
+```
+
+Then build the project:
+```bash
+mage build
+```
+
+This creates the `gt` binary in the project directory.
+
+### Using Go Directly
+
+```bash
+go build -o gt ./cmd/gt
+```
+
+## Testing
+
+Run all tests:
+```bash
+mage test
+```
+
+Run RPN-specific tests:
+```bash
+mage testRPN
+# or
+mage rpn
+```
+
+Run tests with coverage:
+```bash
+go test -coverprofile=coverage.out ./...
+go tool cover -func=coverage.out
+```
+
+## Code Style
+
+- Follow Go best practices (see [Go Code Review Comments](https://github.com/golang/go/wiki/CodeReviewComments))
+- Add comprehensive documentation comments for all exported functions and types
+- Use descriptive variable and function names
+- Keep functions focused and modular
+- Write tests for new functionality
+
+## Pull Request Guidelines
+
+### Before Submitting
+
+1. Ensure all tests pass: `mage test`
+2. Ensure the code builds: `mage build`
+3. Format your code: `go fmt ./...`
+4. Check for linting issues: `golangci-lint run ./...`
+5. Update documentation if needed (README.md, godoc comments)
+
+### PR Description
+
+Include:
+- A clear description of the changes
+- Related issues (if any)
+- Any breaking changes or migration notes
+- Screenshots or examples for UI changes
+
+### Review Process
+
+1. At least one maintainer review is required
+2. All tests must pass
+3. Code coverage should not decrease
+4. Follow the project's coding standards
+
+## Development Workflow
+
+### Typical Workflow
+
+1. Create a branch:
+   ```bash
+   git checkout -b feature/your-feature-name
+   ```
+
+2. Make changes and commit:
+   ```bash
+   git add .
+   git commit -m "Describe your changes"
+   ```
+
+3. Push to your fork:
+   ```bash
+   git push origin feature/your-feature-name
+   ```
+
+4. Open a pull request on GitHub
+
+### Versioning
+
+The project uses semantic versioning. Version bumps are handled by the maintainers.
+
+## Questions?
+
+Open an issue or ask in the pull request discussion.