Adds a griffe-based CI gate over the importable litellm SDK surface. Breaking
changes (removed objects, changed signatures, changed defaults) need a
Conventional Commits declaration, either a bang after the type or a
BREAKING CHANGE: footer. Newly exported top-level names need a feat: or fix:
title so a chore/refactor PR cannot quietly widen the public API.
Scope is deliberately narrower than raw griffe output: litellm.proxy.* is
excluded because the proxy's contract is HTTP rather than Python, and
re-exported stdlib names are excluded because their canonical home is another
package. On a 17-day sample window that took the finding count from 46 to 11,
with all 11 genuine.
