`git diff --cached --name-only` C-quotes filenames with non-ASCII
bytes by default (e.g. a literal "\305\202" for a Polish "ł"), which
doesn't match any real path on disk -- the hook's `[ -f "$file" ]`
check silently skipped those files instead of erroring, so they were
committed as valid LFS pointers but never pinned to IPFS. Use `-z`
(NUL-separated, unquoted) and convert to newlines for the read loop.
Also add an idempotency fast-path to `rad lfs store`/`store_object`:
if a note already exists for an object (e.g. written moments earlier
by `rad lfs precommit`), return its CID directly instead of
re-encrypting. This avoids redundant work, and matters more than that
for private repos specifically: `git push`'s custom-transfer-agent
subprocess chain has no TTY, so without this fast path every push
would fail outright on the passphrase prompt even for objects already
stored interactively at commit time.
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