Add sway/wofi/foot Wayland desktop stack with per-host toggle

New opt-in feature for x86_64 desktops: sway + wofi + foot + swaybg +
swaylock + swayidle + grim + slurp + waybar + wl-clipboard. mako on
arch, dunst on debian (mako isn't packaged for debian).

Files:
- .chezmoi.yaml.tmpl: added data.sway flag (true on miche/byte/kaiser,
  false on Pis). Override per host with ~/.config/chezmoi/features/sway
  or ~/.config/chezmoi/features/no-sway marker files.
- run_once_40-install-sway.sh.tmpl: installs packages if .sway=true,
  exits 0 otherwise. Sets up the marker file.
- dot_config/{sway,foot,wofi,waybar,mako}/: existing configs from miche.

Per-host toggle workflow:
  # On any box, enable sway:
  touch ~/.config/chezmoi/features/sway
  chezmoi apply

  # On a sway-enabled box, disable it:
  touch ~/.config/chezmoi/features/no-sway
  rm ~/.config/chezmoi/features/sway
  chezmoi apply

Currently sway packages are already installed on miche (existed before
this commit). Byte will get them via the new run_once_40 script.
Pis (rye, crouton) are unaffected — install script early-returns.

dot_config/sway/cheatsheet.md

@@ -0,0 +1,48 @@
+# === sway keybinds cheatsheet ===
+# generated from ~/.config/sway/config — `Super` = the Windows key (Mod4)
+# reload this view with:  Super + /
+
+# LAUNCHERS
+Super+Enter         open foot terminal
+Super+D             open wofi app launcher
+Super+E             open thunar (file manager)
+Super+B             open firefox
+Super+/             show this cheatsheet (in foot, via less)
+Super+Shift+E       exit sway (with prompt)
+
+# WINDOW MANAGEMENT
+Super+Q             kill focused window
+Super+F             fullscreen toggle
+Super+V             split toggle (vertical <-> horizontal)
+Super+S             stacking layout
+Super+W             tabbed layout
+Super+Space         floating toggle
+
+# FOCUS / MOVE
+Super+arrows        focus window in direction
+Super+Shift+arrows  move window in direction
+
+# WORKSPACES
+Super+1 .. Super+0     switch to workspace N (0 = 10)
+Super+Tab              toggle back to previous workspace
+Super+Shift+1..0       move focused window to workspace N
+
+# SCREENSHOTS
+Print                region screenshot to file
+Super+Print          full-screen screenshot to file
+Super+Shift+S        region screenshot to clipboard
+
+# SCRATCHPAD
+Super+`              toggle dropdown foot (sway scratchpad)
+
+# RESIZE MODE  (Super+R enters, Esc/Enter exits)
+resize mode:
+    arrows          shrink/grow 30px in direction
+    Esc / Enter     exit resize mode
+
+# LOCK / IDLE
+Ctrl+Alt+L          lock now
+5 min idle          auto-lock
+10 min idle         auto screen-off
+
+# TIP: Super+Shift+C reloads this config without logging out.