Play Framework

docs: clarify nuxt-skel coverage in renovate.json5 (PF-90)

User asked whether the bundled Nuxt skeleton (shipped to new apps via
\`gradle playNewApp -Pfrontend\`) is tracked by Renovate. Answer: yes —
Renovate's npm manager auto-detects resources/nuxt-skel/package.json
and the existing packageRule auto-merges non-major bumps. The link
to -Pfrontend wasn't explicit in the comments, so:

- Header comment now mentions -Pfrontend by name as the consumer of
  this skeleton, so future maintainers see why we care about keeping
  it current.
- Auto-merge rule's justification was misleading (\"tests gating /deploy
  will catch regressions\") — ant test doesn't exercise the Nuxt skel
  and CI never runs pnpm install on it. Replaced with the actual
  reason: blast radius is scaffold-only because already-scaffolded
  apps pin their own versions in pnpm-lock.yaml. Honest framing for
  the next person reading this rule.

No functional change to the rules themselves; the npm manager and
auto-merge for non-major bumps were already there.