docs(sway): note the Wayland greeter login in READMEs

Document the greetd/ReGreet greeter in the top-level README and the T400
and Mac Pro install notes, including that the user account needs a
password set before the greeter can authenticate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -28,6 +28,16 @@ sudo nixos-rebuild switch --flake .#<configuration>
 darwin-rebuild switch --flake .#lyrathorpe-mac
 ```
 
+## Login / greeter
+
+Graphical (Sway) hosts log in through a Wayland greeter — `greetd` running
+ReGreet inside the `cage` kiosk compositor — configured centrally in
+[`lyrathorpe/swaywm.nix`](./lyrathorpe/swaywm.nix), gated on
+`features.swayDesktop.enable`. The greeter is forced to Dvorak to match the
+console and Sway session. Hosts with `features.swayDesktop.enable = false` (the
+WSL work box) keep plain TTY login. The target account needs a password
+(`passwd <user>`) before it can log in.
+
 ## MacBook (Asahi) firmware
 
 The MBP host references `system/modules/firmware/` for Apple peripheral