heather: switch to make-disk-image (qcow2 for YC), drop disko

- Rename muscari -> heather (YC VM). rosemary reserved for future EU server.
- Add hosts/heather/image.nix: YC guest profile + qcow2 build target based on
  nixos/lib/make-disk-image.nix (the nixpkgs-standard cloud image builder).
  partitionTableType=legacy (MBR + single ext4 root, label nixos), GRUB in MBR
  on /dev/vda, virtio drivers, console=ttyS0, cloud-init (Ec2 datasource) so
  poppy ssh key auto-lands from YC metadata, growPartition.
- Drop disko flake input + disk-config.nix (was only for nixos-anywhere runtime
  install; replaced by make-disk-image build-time approach, no kexec).
- Add home/server.nix: minimal headless home for NixOS hosts (no ollama/
  texliveFull/kitty/syncthing/pass). flake mkNixos uses it instead of full ./home.
- k3s.nix: drop dead firewall block (firewall off in image.nix -> trustedInterfaces
  was a no-op). Cilium manages pod networking via BPF.
- image.baseName=heather, configurationLimit=1, remove virtio dups (qemu-guest
  profile already provides most).
- Fix sha256 pin on forgejo mikl.keys (was stale, would break fetchurl).
- home/zsh.nix: source yandex-cloud yc completion.

home/zsh.nix

@@ -87,6 +87,11 @@
         # Right prompt - time in gray
         RPROMPT='%F{240}%D{%H:%M:%S}%f'
         
+        # Yandex Cloud CLI completion
+        if [ -f "$HOME/yandex-cloud/bin/completion.zsh.inc" ]; then
+          source "$HOME/yandex-cloud/bin/completion.zsh.inc"
+        fi
+        
         # Pure prompt customization
         zstyle ':prompt:pure:prompt:success' color green
         zstyle ':prompt:pure:prompt:success' symbol ">"