chore: teach claude about nvd

CLAUDE.md

@@ -32,6 +32,13 @@ If moving things around, you can use `nix eval` before and after changes to chec
 nix eval .#nixosConfigurations.magicman.config.nix.settings.substituters --json | jq
 ```
 
+### Comparing with Running System
+```bash
+# Build and compare against currently running system
+nixos-rebuild build --flake . && nvd diff /run/current-system result
+```
+This shows package version changes, added/removed packages, and closure size differences between what's currently running and the repo configuration.
+
 ### Common Operations
 ```bash
 # Check flake without building