ci: Fix release build - Pass tag_name to GitHub release action

The release workflow now runs from build-main.yml (push to main) instead
of build-tag.yml (push to tag), so GITHUB_REF is refs/heads/main not a
tag ref. softprops/action-gh-release needs tag_name set explicitly.

Prompts:
- "also the release failed"
- (link to failed run showing 'GitHub Releases requires a tag')

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

https://github.com/timabell/gitopolis/actions/runs/24367846445/job/71164504654

Author: timabell

.github/workflows/_release.yml

@@ -51,6 +51,8 @@ jobs:
     - name: Create Release
       uses: softprops/action-gh-release@v2
       with:
+        tag_name: ${{ inputs.release_tag }}
+        name: ${{ inputs.release_tag }}
         body: ${{ steps.changelog.outputs.content }}
         files: |
           gitopolis-linux-x86_64.tar.gz