Support for npm packages or better documentation

I am trying declaratively install an npm package and created

```
description = "OpenSpec CLI (installed via npm during Hermit install)"

binaries = ["OpenSpec-${version}/bin/openspec.js"]

# Basic test to validate the package after installation
test = "bin/openspec --help"

source = "https://github.com/Fission-AI/OpenSpec/archive/refs/tags/v${version}.zip"

repository = "https://github.com/Fission-AI/OpenSpec"

version "0.16.0" {
runtime-dependencies = ["node-24.12.0"]
on install {
run {
cmd = "${HERMIT_BIN}/npm install --prefix ${root} @fission-ai/openspec@${version} --no-fund"
}
}
}
```

I tried different prefix and none of the approaches seem to work:

1. if `--prefix ${root}` - `EACCES: permission denied, mkdir '/Users/iilyak/Library/Caches/hermit/pkg/openspec-0.16.0/node_modules'`
2. if `--prefix ${HERMIT_ENV}/.hermit/node` - the installation succeed but execution fails `Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/Users/iilyak/Library/Caches/hermit/pkg/openspec-0.16.0/OpenSpec-0.16.0/dist/cli/index.js'`
3. if `--prefix ${HERMIT_ENV}` - the installation succeed but execution fails `Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/Users/iilyak/Library/Caches/hermit/pkg/openspec-0.16.0/OpenSpec-0.16.0/dist/cli/index.js'`
4. if I remove `--prefix` - `EACCES: permission denied, mkdir '/Users/iilyak/Library/Caches/hermit/pkg/openspec-0.16.0/node_modules'`

```
ls -la /Users/iilyak/Library/Caches/hermit/pkg/
total 0
drwxr-xr-x 7 iilyak staff 224 Dec 11 12:45 ./
drwxr-xr-x 8 iilyak staff 256 Dec 9 12:36 ../
drwxr-xr-x 3 iilyak staff 96 Dec 9 12:31 hermit@stable/
dr-x------ 9 iilyak staff 288 Dec 11 12:13 node-24.12.0/
dr-x------ 3 iilyak staff 96 Dec 11 12:45 openspec-0.16.0/
```

Reproduce the four npm installation variants using the package definition and commands shown in the issue, recording the permission and module-resolution failures. Determine whether Hermit should support this npm package pattern or document a working configuration; done means a supported installation path or clear documentation for installing and running OpenSpec.

Written by the indexing model from the issue text.