Reference shared actions from Strata repo instead of vendoring

Replace the vendored composite actions (install-cvc5, install-z3,
check-copyright-headers, lint-whitespace, check-lean-import,
check-no-panic) with strata-org/Strata/.github/actions/<name>@main
references, and delete the local copies and their backing scripts. This
removes the duplication called out in review.

lint-whitespace gets an explicit `directory: .` since its Strata default
is `Strata`, which does not exist in this repo's layout.

Keep restore-lake-cache and save-lake-cache local: the Strata versions
hash `**/*.st` in the cache key, which is unstable here because the
cloned Strata dependency drops .st files under .lake between restore and
save, breaking fail-on-cache-miss.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: joehendrix