added tutorial day 0

tutorial/day0/Day0.md

@@ -0,0 +1,48 @@
+# Day 0 — Claude Code Setup
+
+This guide walks you through installing Claude Code on your machine and authenticating so you can start using it.
+
+## Step 1: Install Claude Code
+
+Choose your operating system:
+
+| OS | Guide |
+|----|-------|
+| Windows | [windows.md](windows.md) |
+| Linux | [linux.md](linux.md) |
+| macOS | [mac.md](mac.md) |
+
+Follow the guide for your OS, then come back here for authentication.
+
+---
+
+## Step 2: Verify Installation
+
+After following your OS-specific guide, confirm everything is working:
+
+```bash
+node --version    # Should show v18.x or higher
+claude --version  # Should show the installed Claude Code version
+```
+
+---
+
+## Step 3: Login
+
+<img src="assets/login.png" alt="Claude Code login screen" width="50%">
+
+Run `claude` in your terminal. On first launch, it will ask you to choose a login method.
+
+### Method 1: Subscription (Claude Pro / Max)
+
+- Select **Claude.ai account**
+- Browser opens — sign in and authorize
+- Return to terminal, you're logged in
+
+### Method 2: API Key (Pay-as-you-go)
+
+- Go to [console.anthropic.com](https://console.anthropic.com) → **API Keys** → **Create Key**
+- Select **Anthropic API Key** in the prompt
+- Paste your key (starts with `sk-ant-`)
+
+---