harden and fix: nftables input chain, sudo, agenix, ZFS, NAT priority ...

- Add inet input_filter table to xlab-gateway (policy drop on WAN)
- Restrict NOPASSWD sudo to ldx only; ylw uses password sudo via wheel
- Restructure secrets.nix with admins list, prepare for ylw ed25519 key
- Add ye-lw21 to trusted-users in common.nix
- Remove contradictory relatime=on when atime=off on rpool
- Fix NAT postrouting priority: filter → srcnat
- Remove duplicate nixpkgs.hostPlatform from xlab-gateway hardware-configuration

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

common: disable global flake registry fetch ...

channels.nixos.org is unreachable from CN, causing 25s of
retries on every nix-shell/nix run invocation.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

common: add TUNA mirror as primary Nix substituter, add btop ...

cache.nixos.org has ~1.1s latency from CN. TUNA mirror responds
in ~29ms (38x faster). Set connect-timeout=5 and
stalled-download-timeout=15 to fail fast on unreachable mirrors.

Also add btop to skydick monitoring packages.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

nix: add ldx as trusted-user for deploy-rs unsigned store paths ...

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Initial skyworks infrastructure flake ...

Unified NixOS configuration for skydick (storage server) and
xlab-gateway (lab router). Flat module structure with shared
common/users/ssh modules, agenix secrets, disko, and deploy-rs.

Co-Authored-By: Claude Opus 4.6 <[email protected]>