This is consistent with #9358, but implements support for the GitHub Packages npm registry and, more broadly, for vlt-style https://docs.vlt.sh/cli/registries for any registry.

This PR adds a built-in gh: specifier that resolves against the GitHub Packages npm registry, plus a namedRegistries config key so a project can map its own aliases to arbitrary registries. A project can mix public npm packages and private GitHub Packages (or self-hosted) ones without applying a scope-wide registry override to every @scope/* package.

- pnpm add gh:@acme/private writes "@acme/private": "gh:^1.0.0" and resolves from https://npm.pkg.github.com/.
- pnpm add gh:@acme/private@^1.0.0 (with or without an alias) is also supported. Aliased form writes "my-alias": "gh:@acme/private@^1.0.0".
- Auth comes from the existing per-URL .npmrc mechanism, e.g. //npm.pkg.github.com/:_authToken=${GITHUB_TOKEN}. No new auth surface.
- @github is intentionally not defaulted to https://npm.pkg.github.com/ - hardcoding that would hijack installs of the public @github/* packages on npmjs.org (e.g. @github/relative-time-element) for users without a scope-wide override. Use gh: to install from GitHub Packages, or configure @github:registry=... yourself if that's really what you want.
- Additional named registries (a self-hosted proxy, GitHub Enterprise Server, etc.) can be configured in pnpm-workspace.yaml:

```yml
namedRegistries:
  gh: https://npm.pkg.github.example.com/   # optional: overrides the built-in `gh` alias for GHES
  work: https://npm.work.example.com/
```

- Then work:@corp/lib@^2.0.0 resolves against https://npm.work.example.com/, and the built-in gh alias can be redirected to a GHES host.
- Env-var substitution (${VAR}) is supported in namedRegistries values, mirroring the .npmrc convention.
- Reserved alias names (npm, jsr, github, workspace, catalog, file, git, http, https, link, patch, and related git host shorthands) cannot be redefined as user-named registries - the resolver throws ERR_PNPM_RESERVED_NAMED_REGISTRY_ALIAS at startup rather than silently shadowing another protocol. Malformed URLs throw ERR_PNPM_INVALID_NAMED_REGISTRY_URL at startup too, instead of failing as a confusing 404 during resolution.
- On publish, createExportableManifest strips any named-registry prefix (both the built-in gh: and any user-configured alias) so npm and yarn consumers can still resolve the dependency via their own scope-registry configuration - mirroring the user-facing requirement when installing such a dep without the prefix.

The prefix is gh: rather than github: because github: is reserved by npm-package-arg / hosted-git-info as a git host shorthand (e.g. github:owner/repo) - reusing it would be a deviation from the specs used by the npm CLI. gh: is  shorter, matches vlt's convention, and cannot collide with any existing npm scheme.

Unlike jsr:, gh: (and any other named-registry alias) does not rewrite the package name - gh:@acme/foo resolves @acme/foo from the GitHub Packages registry as-is. This also means npm/yarn consumers see the original name after the prefix is stripped on publish.

---------

Co-authored-by: Zoltan Kochan <z@kochan.io>