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>

system/machine/MacPro31/README.md

@@ -44,6 +44,15 @@ These come up automatically. If a card needs forcing, set
 Wired Ethernet via NetworkManager (from `desktop.nix`) — the Mac Pro has two
 gigabit ports.
 
+## Login
+
+Graphical login via a Wayland greeter — `greetd` running ReGreet inside the
+`cage` kiosk compositor — configured centrally in `lyrathorpe/swaywm.nix` for
+every Sway host (gated on `features.swayDesktop.enable`). The greeter is forced
+to the Dvorak layout to match the console and Sway session. Set the user
+password (`passwd lyrathorpe`) after install, or the greeter cannot
+authenticate. Requires working KMS (radeon/nouveau — see Graphics).
+
 ## Apply
 
 ```sh

system/machine/T400/README.md

@@ -32,6 +32,15 @@ The T400 has switchable graphics (discrete ATI + Intel GMA 4500MHD). Select
 run **Integrated** instead, the Intel `i915` driver takes over with no config
 change and `radeon` stays idle.
 
+## Login
+
+Graphical login via a Wayland greeter — `greetd` running ReGreet inside the
+`cage` kiosk compositor — configured centrally in `lyrathorpe/swaywm.nix` for
+every Sway host (gated on `features.swayDesktop.enable`). The greeter is forced
+to the Dvorak layout to match the console and Sway session. Set the user
+password (`passwd lyrathorpe`) after install, or the greeter cannot
+authenticate. Requires working radeon/i915 KMS (see Graphics).
+
 ## Apply
 
 ```sh