docs: add crab env snapshot/restore to README (#59)

## Summary
- Add "Environment Migration (`crab env`)" section documenting the
snapshot/restore workflow for capturing and restoring dev environments
- Update "New Computer Setup" section with a fast-path restore option
using `crab env restore`

## Test plan
- [ ] Verify markdown renders correctly on GitHub
- [ ] Confirm code blocks and formatting look correct

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

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: MrFlounder

README.md

@@ -290,6 +290,19 @@ crab cheat               # Show cheat sheet
 crab update              # Self-update to latest version
 ```
 
+### Environment Migration (`crab env`)
+
+Capture your entire dev environment and restore it on a new machine. An AI agent explores your machine, builds a comprehensive setup recipe, and packages everything into an encrypted bundle.
+
+```bash
+crab env snapshot              # Agent explores machine → encrypted bundle
+crab env snapshot --dry-run    # Preview what would be captured
+crab env restore --from FILE   # Decrypt + agent sets up new machine
+crab env restore --from URL    # Download and restore from URL
+```
+
+The snapshot captures: Homebrew packages, Node/Python/Go versions, shell config, git identity, editor settings, database schemas, Docker config, cloud tools, project inventory, .env files, and API tokens — all encrypted with a password.
+
 ## Setup Flow
 
 ### 1. Register your project
@@ -460,6 +473,22 @@ With prefix `Ctrl+a`:
 
 ## New Computer Setup
 
+### Fast path: Restore from snapshot
+
+If you have a `crab env` snapshot from your old machine:
+
+```bash
+# 1. Install crabcode
+curl -fsSL https://raw.githubusercontent.com/promptfoo/crabcode/main/install.sh | bash
+
+# 2. Restore everything
+crab env restore --from ~/path/to/snapshot.enc
+```
+
+The restore agent walks through each phase — installing tools, restoring configs, setting up projects — with your approval at each step.
+
+### Manual setup
+
 1. **Install dependencies:**
    ```bash
    brew install tmux yq zip git   # macOS