Update the attic cache URL from s0.koi-bebop.ts.net to s0.neet.dev
and configure sandboxed workspaces to inherit the host's binary cache
settings (substituters, trusted keys, netrc auth via agenix).
Replace nix-serve-only setup with Attic for managed binary caching with
upstream filtering and GC. Move gitea actions runner from host into an
isolated NixOS container with private networking. nix-serve kept alongside
Attic during migration.
