Implement independent format gates for release workflow (issue #86)

- Add XBPS build and sign steps to CI workflow
- Add XBPS publication as independent gate (requires manual trigger)
- Track format availability in release notes
- Update release-footer.md with XBPS install instructions
- Add --available/--withheld arguments to extract_changelog.py
- Attach XBPS artifacts to Gitea release
- Clean up XBPS signing key material after use

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

packaging/release-footer.md

@@ -6,6 +6,13 @@ Install Fenris from its package channel after following the [package setup instr
 sudo apt update && sudo apt install fenris       # Debian / Ubuntu
 sudo dnf install fenris                          # Fedora
 sudo zypper install fenris                       # openSUSE Tumbleweed
+sudo xbps-install fenris                         # Void Linux
+```
+
+For Void Linux, configure the XBPS repository first:
+
+```bash
+sudo xbps-install -S https://git.bongbetic.com/xavierk/Fenris-xbps/raw/branch/stable/x86_64
 ```
 
 ## Verify downloads