gamemode: global auto-trigger via niri window watch

Add scripts/gamemode-watch.py, launched at niri startup, which subscribes
to niri's event stream and toggles gamemode whenever a game window
(steam_app_*, RuneLite, gamescope) is open — so Steam games trigger
gamemode with zero per-game setup instead of needing gamemoderun %command%.

'winwatch' is a third reference-counted source alongside feral + manual.
Pure Watcher class is unit-tested; SIGTERM/finally guarantee cleanup so
the watcher stopping never leaves gamemode stuck on.

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

install.sh

@@ -54,6 +54,7 @@ ln -sf "$(pwd)/scripts/waybar-restart.sh" ~/.local/bin/waybar-restart
 ln -sf "$(pwd)/scripts/screenshot.sh" ~/.local/bin/screenshot
 ln -sf "$(pwd)/scripts/volume.sh" ~/.local/bin/volume
 ln -sf "$(pwd)/scripts/gamemode-session.sh" ~/.local/bin/gamemode-session
+ln -sf "$(pwd)/scripts/gamemode-watch.py" ~/.local/bin/gamemode-watch
 
 echo "==> Enabling systemd user services"
 mkdir -p ~/.config/systemd/user