Implement git-remote-freenet helper binary

[sanity]

Problem

The git remote helper binary that lets git natively use freenet: URLs.

What needs to happen

git-remote-freenet implements the git remote helper protocol:

• Read commands from stdin: capabilities, list, fetch, push.
• Write responses to stdout per the protocol.
• Must be on PATH; git invokes it automatically when it sees a freenet: URL.

list

Read the repo contract via WS GET, return the refs from RepoState.refs.

fetch

For each commit requested:

1. Walk RepoState.object_index, pull SinglePack bundles whose pack contains commits in our want set.
2. For each pack, GET the pack contract by id (pack_contract_id helper already exists).
3. Verify BLAKE3(pack_bytes) matches what we expect, then unpack into .git/objects.

push

1. Build a packfile of new objects with git pack-objects.
2. PUT the pack as a new pack-contract instance.
3. Sign and submit a delta to the repo contract that adds the bundle to object_index and updates refs[ref] to the new commit.
4. Subscribe to the repo contract; wait for the network to reflect our update is the visible head.
5. If the visible head is someone else's update (race), surface "superseded — please rebase" via stderr and exit non-zero.

Phase 1.0 simplifications

• SinglePack only (no ChunkedPack support; reject contracts that have unfetchable ChunkedPack records with a clear message).
• Single-writer; the helper signs as the loaded identity.

Pointers

• freenet-git-types::signing has all the sign helpers already.
• freenet-git/src/ids.rs has pack_contract_id and repo_contract_id.
• The WS API client work in Wire up WS API publish in freenet-git create #1 will be reused here.

[AI-assisted - Claude]

[sanity]

Implemented in 4864da5. Demonstrated end-to-end against the live network: freenet-git create, git push freenet::<id> main, git clone freenet::<id>, and git pull all work. The actual repo from the demo is at freenet:9BfYws4gvjPVVxG5UNDVkTBYmiyTJX1yKjv4bqhjezh.

[AI-assisted - Claude]