chore: harden registry resolution

[MohamedH1998]

What & why

Checklist

• A maintainer approved this work (lgtm+ on my issue or discussion), or I'm on the team
• Correct tier (framework / starter / registry) per the boundary test
• Edited packages/nimbus-starter-source/, not the templates branch
• Changeset added (create-nimbus-docs changeset if the starter changed)
• pnpm typecheck, pnpm -r test, and pnpm templates:check all green

First time here?

Nimbus works from issues and discussions, not drive-by PRs, so unapproved PRs from
outside the team are closed automatically. If that's you, open an
issue (bug or fix proposal)
or a discussion (feature) instead —
once a maintainer approves you there, your PRs stay open. See
CONTRIBUTING.md.

[pkg-pr-new]

pnpm add https://pkg.pr.new/@cloudflare/create-nimbus-docs@113

pnpm add https://pkg.pr.new/@cloudflare/nimbus-docs@113

commit: 9a3ccf8