docs: add README, CONTRIBUTING, and CHANGELOG

- README with project overview, architecture diagram, and quick start
- CONTRIBUTING with development setup and code style guidelines
- CHANGELOG starting at 0.1.0 (unreleased, under active development)

CONTRIBUTING.md

@@ -0,0 +1,48 @@
+# Contributing to Pipely
+
+Thank you for your interest in contributing.
+
+## Getting Started
+
+1. Fork the repository
+2. Clone your fork: `git clone git@onixbyte.dev:<your-org>/pipely.git`
+3. Create a branch: `git switch -c feature/your-feature`
+
+## Development
+
+### Backend (Go)
+
+```bash
+go run cmd/server/main.go      # run the server
+go test ./...                   # run tests
+go vet ./...                    # lint
+```
+
+### Frontend (React)
+
+```bash
+cd web
+pnpm install
+pnpm dev                        # dev server with HMR and API proxy
+pnpm build                      # production build
+```
+
+## Code Style
+
+- **Go**: Standard `gofmt` formatting. Run `go fmt ./...` before committing.
+- **TypeScript**: ESLint + Prettier. Run `pnpm lint` before committing.
+- **Spelling**: British English throughout (e.g. `colour`, `centre`, `optimise`, `artefact`).
+- **Commit messages**: Follow [Conventional Commits](https://www.conventionalcommits.org/).
+
+## Pull Requests
+
+- Keep changes focused and minimal
+- Ensure `go build ./...` and `pnpm build` pass
+- Update documentation if your change affects the API
+
+## Reporting Issues
+
+Use the issue tracker. Include:
+- Steps to reproduce
+- Expected and actual behaviour
+- Environment details (Go version, PostgreSQL version, OS)