Add v2026.2.0 release to metainfo, simplify CI workflow

- Metainfo is managed upstream (updated before tagging)
- CI no longer modifies metainfo; just copies manifest and sources to Flathub
- Flathub manifest installs metainfo from git source
- Permissions reverted to read-only

flatpak/app.yaak.Yaak.metainfo.xml

@@ -52,5 +52,6 @@
   </screenshots>
 
   <releases>
+    <release version="2026.2.0" date="2026-02-10" />
   </releases>
 </component>