Previously the pre-commit hook shelled out to `rad lfs store` once per
staged LFS file, each a separate process. For private repos, each of
those needs a keystore passphrase for an ECDH key-agreement operation,
so a multi-file commit meant a separate interactive passphrase prompt
per file -- observed live against the blog repo (9+ prompts in a row).
Add `rad lfs precommit`, which reads every staged file's oid/size/path
from stdin and processes them all in one process, loading the signer
at most once and reusing it across the batch. `store::run` now
delegates to a shared `store_object` helper that both it and
`precommit::run` call.
c070aac6fe