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

README.md

@@ -100,6 +100,20 @@ The Nix flake also provides a development shell with all dependencies:
 nix develop
 ```
 
+## Building a Flatpak
+
+NoMines ships a Flatpak manifest (`io.github.bemagri.nomines.yml`) that builds
+the gtkmm stack (libsigc++, glibmm, gtkmm) against the GNOME 46 runtime:
+
+```bash
+flatpak install -y flathub org.gnome.Platform//46 org.gnome.Sdk//46
+flatpak-builder --user --install --force-clean build-flatpak io.github.bemagri.nomines.yml
+flatpak run io.github.bemagri.nomines
+```
+
+Note: the manifest's `commit` must match the pushed `v1.0.0` tag (update it if
+the history is rebased before pushing).
+
 ## Contributing
 
 Contributions are welcome! Please feel free to submit a Pull Request.