docs: add Requirements section and clarify flow

- Add Requirements section making clear Nix must be installed first
- Link to Determinate Nix docs
- Include install command for those without Nix
- Add note to Plugins section: complete Quick Start first

Addresses user feedback about unclear ordering.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -9,6 +9,7 @@
 ## Table of Contents
 
 - [What You Get](#what-you-get)
+- [Requirements](#requirements)
 - [Why Nix?](#why-nix)
 - [Quick Start](#quick-start)
 - [How It Works](#how-it-works)
@@ -43,6 +44,20 @@ You talk to Telegram, your Mac does things.
 
 ---
 
+## Requirements
+
+1. **macOS** (Apple Silicon or Intel)
+2. **[Determinate Nix](https://docs.determinate.systems/determinate-nix/)** installed on your machine
+
+That's it. The Quick Start will guide you through everything else.
+
+> **Don't have Nix yet?** The [Quick Start](#quick-start) agent copypasta will install it for you, or you can run:
+> ```bash
+> curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install
+> ```
+
+---
+
 ## Why Nix?
 
 You've probably installed tools before. Homebrew, pip, npm - they work until they don't.
@@ -202,6 +217,8 @@ All state lives in `~/.clawdbot/`. Logs at `/tmp/clawdbot/clawdbot-gateway.log`.
 
 ## Plugins
 
+> **Note:** Complete the [Quick Start](#quick-start) first to get Clawdbot running. Then come back here to add plugins.
+
 Plugins extend what Clawdbot can do. Each plugin bundles tools and teaches the AI how to use them.
 
 ### First-party plugins