ci: validate renovate config in lint workflow

[cj848]

Motivation

• renovate.json now carries version-line lock rules (ci: split renovate dependency updates by version line #1212) that the Renovate bot reads on every run. A config error (bad option name, malformed regex, schema change) is currently only discovered after the bot runs and fails or silently misbehaves.
• The official Renovate CLI ships renovate-config-validator, which validates schema, option names, and regex syntax offline. Running it in CI catches broken configs in the PR that introduces them.

Modifications

• New renovate-config job in .github/workflows/lint.yaml (alongside commit-lint and ktlint):
  • actions/checkout@v7 -> actions/setup-node@v7 (Node 24.x, same as the commit-lint job) -> npx -y -p renovate renovate-config-validator.
  • Purely additive: 11 lines, no changes to existing jobs, no package.json change (the validator is fetched on demand, so the commit-lint npm install stays lean).

Result

• Every PR to main/develop now validates renovate.json with the official validator — a config mistake fails the Lint workflow instead of surfacing as bot misbehavior later.
• Verified locally against the current renovate.json: Config validated successfully.

Closes

• N/A