Add flatpak build plumbing

- io.github.bemagri.nomines.yml: builds libsigc++/glibmm/cairomm/pangomm/gtkmm
  from tarballs against the GNOME 46 runtime, then the app from the v1.0.0 tag
- Icon: app-id-named PNGs (128/512) for flatpak export, svg kept for native
  installs; desktop file Icon updated
- README/AGENTS.md: flatpak build instructions and version traps

resources/io.github.bemagri.nomines.desktop

@@ -4,7 +4,7 @@ GenericName=Minesweeper Game
 Comment=Classic minesweeper game with modern GTK UI
 Keywords=game;puzzle;mine;
 Exec=nomines
-Icon=nomines
+Icon=io.github.bemagri.nomines
 Terminal=false
 Type=Application
 Categories=Game;LogicGame;