Add a 'domain' option to the NixOS module and JSON config that
controls the domain suffix used for internal DNS names. Defaults to
'hyprspace' for backward compatibility. Supports multi-label domains
(e.g. 'vpn.internal').
- nixos/settings.nix: add 'domain' option
- config/config.go: read Domain from config, default to 'hyprspace'
- dns/server.go: use config.Domain instead of hard-coded value
- docs: document the new option
- dns/server_test.go: add tests for domainSuffix
* README: document basic build/setup steps
* document dev setup and how the config schema works
* set up for use with direnv
* mention direnv in the dev setup docs
* README: mention developer docs
* provide some contributor infos
