Fix systemd 203/EXEC: use absolute path for claude binary
Summary
- Agent services fail with
status=203/EXECbecauseclaudeis not on the systemd service PATH -
claudeCommandnow defaults to the absolute nix store path vialib.getExe cfg.claudePackage - Adds
claudePackageoption (default = pkgs.claude-code) for overriding the CLI package