docs(repo): add development setup guide for Windows and Linux

docs/setup.md

@@ -0,0 +1,43 @@
+# Development Setup
+
+Verify everything works after setup:
+
+```bash
+python scripts/check_all.py --all
+```
+
+## Required tools
+
+- Python 3.9+
+- g++ / gcc (C++17/20 and C17)
+- Node.js (JavaScript solutions)
+- Rust via `rustup` (optional, local study only; see [Language Policy](language-policy.md))
+- Git and the [GitHub CLI](https://cli.github.com/) (`gh`)
+
+## Windows
+
+```powershell
+choco install mingw -y
+winget install -e --id Python.Python.3.12
+winget install -e --id OpenJS.NodeJS.LTS
+winget install -e --id Rustlang.Rustup
+```
+
+Open a **new** terminal window afterward: PATH changes do not reach windows already open.
+
+## Linux (Debian/Ubuntu)
+
+```
+sudo apt update
+sudo apt install -y build-essential python3 nodejs npm
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+```
+
+## One-time per clone
+
+```
+git config core.hooksPath scripts/hooks
+git update-index --chmod=+x scripts/hooks/pre-commit scripts/hooks/pre-push
+```
+
+See [Testing](testing.md) for what each check does, and [Coding Standards](coding-standards.md) for the rules they enforce.