feat(rust): add peer autosync service for non-bare repos

[rrvsh]

Summary

• add a new git-peer-sync Rust crate that watches a repository, auto-commits local changes, and syncs peers via pull --rebase + push
• implement rebase conflict handling that writes a MERGE_CONFLICT file with pull stderr, unmerged files, status, and conflict diff summary
• package the tool in Nix as packages.git-peer-sync with wrapped runtime dependencies (git, openssh) and sandbox test support
• include crate tests that validate push/pull behavior against non-bare repos and conflict-file creation

Verification

• cargo test --manifest-path rs/Cargo.toml -p git-peer-sync
• cargo clippy --manifest-path rs/Cargo.toml -p git-peer-sync --all-targets
• nix build .#git-peer-sync
• manual temp-dir runs of sync-once for both successful sync and rebase-conflict paths