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

meson.build

@@ -37,10 +37,22 @@ executable('nomines',
     install : true
 )
 
-# Install icon
+# Install icon (named after the app id so flatpak export picks it up;
+# PNG sizes are required by flatpak/flathub icon validation)
 install_data(
     'resources/nomines.svg',
-    install_dir: join_paths(get_option('datadir'), 'icons/hicolor/scalable/apps')
+    install_dir: join_paths(get_option('datadir'), 'icons/hicolor/scalable/apps'),
+    rename: 'io.github.bemagri.nomines.svg'
+)
+install_data(
+    'resources/nomines-128.png',
+    install_dir: join_paths(get_option('datadir'), 'icons/hicolor/128x128/apps'),
+    rename: 'io.github.bemagri.nomines.png'
+)
+install_data(
+    'resources/nomines-512.png',
+    install_dir: join_paths(get_option('datadir'), 'icons/hicolor/512x512/apps'),
+    rename: 'io.github.bemagri.nomines.png'
 )
 
 # Install desktop file