fix(corp-mirrors): resolve @pnpm from npmjs registry (#120)

## Summary

- Scope only the `@pnpm` package to the npmjs registry so pnpm's
package-manager self-install resolves canonically, while every other
install keeps flowing through the corp artifactory mirror.

## Why

pnpm 11 hardened its package-manager self-install: when a repo pins
`packageManager` (e.g. `pnpm@10.23.0`), pnpm fetches it as `@pnpm/exe`
and asserts an **integrity-only** resolution. The artifactory npm mirror
rewrites the tarball host to its own domain, so the resolution carries a
`tarball` field alongside `integrity` and the assertion throws. The
result is that every `pnpm` invocation in such a repo fails with:

```
The packageManager dependency "@pnpm/exe@10.23.0" in pnpm-lock.yaml must use a registry package path and an integrity-only resolution
```

This is not the pnpm version, the pin format, or
`manage-package-manager-versions`. Each variable was isolated to confirm
the sole trigger is the mirror's rewritten tarball host. Pinning the
`@pnpm` scope to npmjs makes the package manager resolve from the
canonical registry, where the resolution is integrity-only and the
assertion passes.

## Test plan

- [x] `nixos-rebuild switch --flake .#wsl` applies cleanly
- [x] `pnpm --version` in a repo pinning `packageManager` returns the
pinned version instead of erroring
- [x] `pnpm install`, `pnpm lint`, and a Dockerfile `pnpm` build all run
- [x] Non-`@pnpm` installs still resolve through the artifactory mirror

Author: hakula139

.cspell/words.txt

@@ -161,6 +161,7 @@ nixsw
 nixup
 nlsv
 nofile
+npmjs
 nslookup
 nspr
 numtide

home/modules/corp-mirrors/default.nix

@@ -116,9 +116,12 @@ in
         # ----------------------------------------------------------------------
         # Node.js Development
         # ----------------------------------------------------------------------
+        # Resolve @pnpm from npmjs. pnpm 11 rejects the mirror's rewritten
+        # tarball host when self-installing a pinned packageManager.
         ".npmrc".text = lib.mkForce ''
           manage-package-manager-versions=true
           registry=${npmMirror}
+          @pnpm:registry=https://registry.npmjs.org/
         '';
 
         # ----------------------------------------------------------------------