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/config.d/20-autostart.conf

@@ -0,0 +1,15 @@
+# === ~/.config/sway/config.d/20-autostart.conf ===
+# background services started when sway comes up.
+
+# polkit agent — needed for gparted, mount UIs in thunar, etc.
+exec /usr/libexec/polkit-gnome-authentication-agent-1
+
+# clipboard — start clipmenud directly (it watches the clipboard internally)
+# Drop the wl-paste --watch wrapper to avoid bar noise if the socket isn't ready.
+exec clipmenud
+
+# xdg-desktop-portal — required for native file/screen sharing in Firefox/Chromium.
+# Skip if services aren't enabled; the user can enable with:
+#   systemctl --user enable --now xdg-desktop-portal xdg-desktop-portal-wlr
+# Until then, those systemctl lines are quiet no-ops.
+exec sh -c 'systemctl --user start xdg-desktop-portal xdg-desktop-portal-wlr 2>/dev/null || true'