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>

niri/config.kdl

@@ -45,6 +45,7 @@ spawn-at-startup "hypridle"
 spawn-at-startup "wl-paste" "--watch" "cliphist" "store"
 spawn-at-startup "wlsunset" "-l" "49.2" "-L" "-123.1"
 spawn-at-startup "xwayland-satellite"
+spawn-at-startup "gamemode-watch"
 
 binds {
     Mod+Q repeat=false { close-window; }