The packages job compiled dbx-cli/dbx-mcp cargo targets with no cache at
all. agent-rust only had rust-cache, so every lock change recompiled the
bundled duckdb C++ library. The GHA sccache fallback for fork PRs wrote
nothing (0% hits, all writes erroring under the v2 cache service) while
main only populates S3, so forks now build without a wrapper and lean on
the rust-cache restore from main.