Add a scheduled (nightly/weekly) CI run

[Amayyas]

What

Add a schedule: cron trigger to the CI workflow (or a dedicated one) so dependency drift (e.g. a new RuboCop release introducing new offenses, a new Ruby patch version) is caught proactively instead of surprising the next real PR.

Acceptance criteria

• Scheduled workflow added (suggest weekly, e.g. Monday morning)
• Runs the same checks as regular CI (tests matrix, RuboCop, bundler-audit)