refactor(deploy): rename system user lamassu → bitspire

System-level half of the lamassu → bitspire rebrand the rest of dev
already did at the path / service / package layers. Touches user/group
declarations, every systemd `User=` block, the udev rules filename, all
chown calls in flake.nix + live.nix, the displayManager autoLogin user,
the trusted-users nix entry, provision-atm.sh's ATM_USER, plus README +
CLAUDE.md doc references.

In-place migration for the Sintra dev unit (which auto-pulls dev at
04:00) lives in `system.activationScripts.bitspire-user-migration` and:
- copies `/home/lamassu/.ssh/authorized_keys` → `/home/bitspire/` once,
  so SSH access survives the rename
- recursively chowns `/var/lib/bitspire` to the new bitspire UID on
  every boot — cheap no-op once done, but covers the case where the
  data dir was written by the now-removed lamassu UID
- leaves `/home/lamassu/` in place as evidence; operator can `rm -rf`
  after confirming bitspire login works

Recovery path if the migration breaks SSH access: root key is still in
configuration.nix:142-144 (padreug@gizmo), so ssh root@<host> works.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy/nixos/live.nix

@@ -175,7 +175,7 @@ in
     if [ ! -f /var/lib/bitspire/.env ]; then
       cp ${envTemplate} /var/lib/bitspire/.env
       chmod 600 /var/lib/bitspire/.env
-      chown lamassu:lamassu /var/lib/bitspire/.env
+      chown bitspire:bitspire /var/lib/bitspire/.env
     fi
   '';
 
@@ -189,7 +189,7 @@ in
     before = [ "bitspire.service" ];
     serviceConfig = {
       Type = "oneshot";
-      User = "lamassu";
+      User = "bitspire";
       Environment = "DISPLAY=:0";
       ExecStart = "${pkgs.bash}/bin/bash -c '${pkgs.xorg.xrandr}/bin/xrandr --output eDP-1 --off; sleep 1; ${pkgs.xorg.xrandr}/bin/xrandr --output eDP-1 --auto'";
     };