feat(release): attach native Arch package and repackage for AUR

Update release workflow to detect and attach the native Arch package.
Switch the binary AUR recipe to repackage the .pkg.tar.zst. Adjust
checksums and packaging steps so the output carries only the native
payload without bundling upstream package metadata.

- Attach the native .pkg.tar.zst to Gitea releases.
- Rework binary AUR recipe to extract usr from the native package.
- Update README to document using the native Arch package.

README.md

@@ -60,7 +60,7 @@ Install Gitty from the AUR with an AUR helper:
 yay -S gitty-desktop
 ```
 
-To install the prebuilt AppImage instead of compiling from source:
+To install the prebuilt native Arch package instead of compiling from source:
 
 ```bash
 yay -S gitty-desktop-bin
@@ -75,8 +75,9 @@ makepkg -si
 ```
 
 The source recipe downloads the public Gitea release archive and builds Gitty.
-The `-bin` recipe installs the prebuilt AppImage. The release pipeline updates
-both packages' versions, checksums, and `.SRCINFO` files.
+The `-bin` recipe repackages the native `.pkg.tar.zst` release artifact. The
+release pipeline updates both packages' versions, checksums, and `.SRCINFO`
+files.
 
 ---