Configure all NixOS machines to use the internal harmonia binary cache:
- Add nix-cache.johnogle.info as substituter
- Add harmonia signing public key to trusted-public-keys
- Enable fallback for local builds when cache unreachable
- Set 5s connect-timeout for faster fallback
Refs: x-qdkuu
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Create roles/common.nix containing shared configuration between NixOS and
Darwin: timezone, base packages (git, glances, pciutils, tree, usbutils, vim),
nix settings (experimental-features, max-jobs, trusted-users), gc config,
and allowUnfree setting.
Both roles/default.nix and roles/darwin.nix now import common.nix and only
contain platform-specific configuration.
