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>

de.pc_heini.HeiniRemoteAgent.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <!-- HeiniRemote "Fernblick": remote screen inside a screen, PC-Heini logo
+       yellow (#f6db3e) on an anthracite tile (variant B2 of the logo drafts). -->
+  <rect width="32" height="32" rx="7" fill="#1d1e20"/>
+  <rect x="0.5" y="0.5" width="31" height="31" rx="6.5" fill="none" stroke="#f6db3e" stroke-opacity="0.25"/>
+  <rect x="7" y="8.5" width="18" height="13" rx="2.5" fill="none" stroke="#f6db3e" stroke-width="2.6"/>
+  <rect x="14.5" y="13.5" width="7.5" height="5.5" rx="1.2" fill="#f6db3e"/>
+  <rect x="12" y="23.5" width="8" height="2" rx="1" fill="#f6db3e" opacity="0.85"/>
+</svg>