CI: cache opam dependencies, refresh weekly (#472) * CI: cache opam dependencies, refresh weekly (#471) Add restore_cache/save_cache steps to the CircleCI build command, keyed by OCaml version and week number. The cache is restored before installing opam deps and saved afterwards, so subsequent CI runs in the same week skip the slow package installation. Also make setup-opam idempotent: skip the opam repo add/remove if the github repo is already present (as it would be in a cache hit). Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com> * Innocuous change to test opam caching --------- Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>