fix(MM-65): address PR review — toolchain parity and CLAUDE.md
- Use fromRustupToolchainFile instead of stable.latest so nix build
and nix develop always resolve the same Rust toolchain
- Remove crane.inputs.nixpkgs.follows (crane has no nixpkgs input;
it takes pkgs as an argument)
- Add nix build .#relay command to CLAUDE.md Commands