npm with "nodejs system" warns about "No version is set"

When I configure nodejs to "system" (nodejs 25.9.0 via homebrew) and call `npm` I get a warning about `No version is set for nodejs`:

```
> /opt/homebrew/bin/node -v
v25.9.0

> /opt/homebrew/bin/npm -v
11.12.1

> ~/.asdf/installs/nodejs/20.11.1/bin/npm -v
10.2.4

> asdf list nodejs
*20.11.1

> asdf set -u nodejs 20.11.1
> npm -v
10.2.4

> asdf set -u nodejs system
> npm -v
No version is set for nodejs; please run `asdf set [options] nodejs `
11.12.1
```

Start by reproducing the documented `asdf set -u nodejs system` and `npm -v` commands, then trace how the plugin handles the system version and npm executable. Done means using the system Node.js and npm reports the version without the “No version is set for nodejs” warning.

Written by the indexing model from the issue text.