docs: note how to trigger a rebuild (and trigger one)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018jkHZhr7ZzHGGuE2bJQmGa

README.md

@@ -100,3 +100,5 @@ StatusNotifier names for the tray icon.
 | `heiniremote-wrapper.sh` | Entry point: host-shell `$SHELL` override + config adoption. |
 | `host-shell.sh` | `flatpak-spawn --host` bridge for the PTY console. |
 | `de.pc_heini.HeiniRemoteAgent.*` | Desktop file, AppStream metainfo (version stamped by CI), icon. |
+
+Builds track the newest upstream `main` commit; a push to this repo (even a README change like this line) triggers a fresh build.