Add `make audit` target running `cargo audit` with an advisory ignore for
RUSTSEC-2025-0141 (bincode, tracked by #636). Wire audit into `make pre-pr`
and `make pre-pr-server`. Add conditional cargo audit to the pre-commit hook
(triggers only when Cargo.toml/Cargo.lock are staged).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>