Adds Git LFS support for repositories, with large file content stored on
each contributor's own local IPFS node instead of a seed-hosted server:
- `rad lfs init`: one-time setup (transfer-agent config, pre-commit hook,
notes-ref push/fetch refspecs, IPFS daemon reachability check).
- `refs/notes/rad-lfs` carries the oid -> CID mapping, replicating with
the repository via Radicle's existing ref sync (verified against
`references_of` in crates/radicle/src/storage/git.rs: every ref is
replicated except refs/tmp/heads/*, so no protocol changes were needed).
- `rad seed`/`rad unseed` now pin/unpin a repo's known LFS objects in IPFS,
mirroring Radicle's existing "seeding = keep a full copy" model, and
warning (not failing) if no local IPFS daemon is reachable.
The actual byte transfer is handled by a separate binary, rad-lfs-transfer
(added here as a submodule), which implements the Git LFS custom-transfer-
agent protocol against a local IPFS daemon. See LFS-IPFS.md for the full
design and setup instructions.
Building and using `rad` for anything other than `rad lfs` requires no
IPFS dependency at all; `rad lfs init` checks for a local daemon upfront
and fails with an actionable message rather than proceeding silently.
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