Add Pi theme based on system theme

Based entirely on the work of @robzolkos in #5711. That attempt was
adapted to better fit the standard templating system while also adding
color mixing for use in other themes.

migrations/1779242346.sh

@@ -0,0 +1,5 @@
+echo "Replace Pi's Omarchy light/dark extension with the generated omarchy-system theme"
+
+rm -f "$HOME/.pi/agent/extensions/omarchy-system-theme.ts"
+omarchy-theme-refresh
+omarchy-theme-set-pi --activate