docs: add CONTRIBUTING.md with setup and submission guide

CONTRIBUTING.md

@@ -0,0 +1,110 @@
+# Contributing to Anvil
+
+Thanks for your interest in contributing to Anvil! This guide will help you get set up and explain how we work.
+
+## Prerequisites
+
+- [Node.js](https://nodejs.org/) (v20+)
+- [pnpm](https://pnpm.io/) (v9+)
+- [Rust](https://rustup.rs/) (stable toolchain)
+- [Tauri CLI](https://v2.tauri.app/start/prerequisites/) (`cargo install tauri-cli`)
+
+### macOS
+
+Xcode Command Line Tools are required:
+
+```bash
+xcode-select --install
+```
+
+### Linux
+
+Install system dependencies for Tauri:
+
+```bash
+sudo apt install libwebkit2gtk-4.1-dev build-essential curl wget file \
+  libssl-dev libayatana-appindicator3-dev librsvg2-dev
+```
+
+## Getting Started
+
+```bash
+# Clone and install dependencies
+git clone https://github.com/zdenham/anvil.git
+cd anvil
+pnpm install
+
+# Start development (builds all workspaces + launches Tauri)
+pnpm dev
+```
+
+The `pnpm dev` command starts the agents, SDK, migrations, and sidecar workspaces in watch mode alongside the Tauri development server.
+
+## Project Structure
+
+```
+anvil/
+  src/              # Frontend (React + TypeScript)
+  src-tauri/        # Tauri backend (Rust)
+  core/             # Shared types, adapters, SDK, services
+  agents/           # Agent workspace package
+  server/           # Backend server (gateway, routes, migrations)
+  api/              # Fastify API server
+  sidecar/          # Sidecar process
+  migrations/       # Database migrations
+  landing/          # Marketing site
+```
+
+## Running Tests
+
+```bash
+# Unit + integration tests
+pnpm test
+
+# Watch mode
+pnpm test:watch
+
+# UI component tests
+pnpm test:ui
+
+# E2E tests (requires a running dev server)
+pnpm test:e2e
+
+# Critical-path E2E only
+pnpm test:e2e:critical
+```
+
+## Linting
+
+```bash
+pnpm lint        # Check for issues
+pnpm lint:fix    # Auto-fix
+```
+
+## Submitting Changes
+
+1. Fork the repository and create a branch from `main`
+2. Make your changes in small, focused commits
+3. Add tests for new functionality
+4. Ensure `pnpm test` and `pnpm lint` pass
+5. Open a pull request with a clear description of what changed and why
+
+## Code Style
+
+- TypeScript strict mode is enforced
+- Use Vitest for unit and integration tests
+- Prefer named exports over default exports
+- Keep files focused and under 250 lines (see `docs/agents.md`); break larger files into submodules
+
+## Reporting Issues
+
+Open an issue on GitHub with:
+
+- Steps to reproduce
+- Expected vs. actual behavior
+- OS and Anvil version
+- Relevant logs (from `logs/dev.log` if running in dev mode)
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the [MIT License](LICENSE).