fix: skip pre-push checks on branch deletion

The pre-push hook runs lint/typecheck/test/build checks on every push,
including `git push origin --delete <branch>`. Branch deletion does not
push any code, so verification checks are unnecessary and block the
delete operation.

Detect deletion pushes by reading stdin (local sha is all zeros for
deletes) and exit early.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>