sway: per-host overrides via host.conf and session-env

Adds a host_sway_dotfiles mechanism mirroring host_i3_dotfiles: hosts
drop a host.conf into ~/.config/sway/ to override sway settings, and
optionally a session-env file sourced by start-sway for per-host
environment variables. The base sway/config defines $terminal and
includes ~/.config/sway/host.conf; an empty placeholder ships with the
base profile so the include never warns on hosts without overrides.

nymph gets its own host.conf wrapping alacritty in prime-run (NVIDIA
PRIME offload), bumping gaps/corner_radius, and pinning workspaces to
eDP-1 / DP-1, plus a session-env exporting SWAY_UNSUPPORTED_GPU=1 and
WLR_DRM_DEVICES for the hybrid Intel+NVIDIA setup.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

dotfiles/desktop/.config/sway/config

@@ -10,6 +10,9 @@ set $down j
 set $up k
 set $right l
 
+# Default terminal (hosts can override via host.conf — e.g. nymph wraps it in prime-run)
+set $terminal alacritty
+
 # Workspaces (waybar formatta i numeri in romani via format-icons)
 set $ws1  "1"
 set $ws2  "2"
@@ -62,6 +65,9 @@ gaps outer 4
 smart_gaps on
 smart_borders on
 
+# Per-host overrides (terminal wrapper, gaps tweaks, workspace→output bindings, …)
+include ~/.config/sway/host.conf
+
 # Window rules
 for_window [app_id="(?i)polkit"] floating enable
 for_window [class="(?i)polkit"] floating enable
@@ -108,7 +114,7 @@ exec swayidle -w \
     before-sleep '~/.config/sway/scripts/lockscreen'
 
 # Terminal
-bindsym $mod+Return exec alacritty
+bindsym $mod+Return exec $terminal
 
 # Emacs client
 bindsym $mod+Shift+e exec emacsclient -c -n