Create contributing.md

Author: Chintan Patel

contributing.md

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+# Contributing
+
+## Setup
+
+```bash
+git clone https://github.com/Chintanpatel24/arrowcode.git
+cd arrowcode
+bun install
+bun run ci
+```
+
+## Checks before PR
+
+```bash
+bun run typecheck
+bun run test:smoke
+bun run src/index.ts --help
+bun run src/index.ts --banner
+```
+
+## Guidelines
+
+- Keep file tools sandboxed via `Workspace`
+- Prefer workspace-local session/checkpoint data over creating `~/.arrowcode` unless install/setup
+- No emoji in TUI output
+- Update `GUIDE.md` / `docs/` when adding commands or flows
+- Add smoke coverage for new critical paths when practical
+
+## Layout
+
+See [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) and [GUIDE.md](GUIDE.md).