Adds a CI workflow that blocks PRs introducing commits with
unmapped author emails. Checks each new commit's author email
against AUTHOR_MAP in scripts/release.py — GitHub noreply emails
auto-pass, but personal/work emails must be mapped.
Also adds --strict and --diff-base flags to contributor_audit.py
for programmatic use. --strict exits 1 when new unmapped emails
are found; --diff-base scopes the check to only flag emails from
commits after a given ref (grandfathers existing unknowns).
Prevention for the 97-unmapped-email gap found in the April 2026
contributor audit.
