feat: Flatpak packaging for the HeiniRemote agent

Bundles the agent as de.pc_heini.HeiniRemoteAgent on the freedesktop 24.08
runtime, mirroring the o3de-flatpak CI approach (bwrap-free build, OSTree
repo force-pushed to pages, single-file bundle attached to a Gitea release).
Compiles the agent from upstream source (newest v* tag, else main).

Sandbox notes: screen capture/input via portals need no special permission;
the PTY console uses flatpak-spawn --host for a real host shell, and a native
install's agent.toml is adopted read-only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,6 @@
+build-dir/
+repo/
+publish/
+upstream/
+dist/
+*.flatpak