Replace GitHub Actions with Gitea Actions

- Remove all GitHub Actions workflows (21 files)
- Remove GitHub problem matchers
- Add Gitea build workflow (push/PR to main)
- Add Gitea release workflow (version tags)
- Use pixi for dependency management
- Configure ccache for build acceleration
- Use 16 build threads for debian-host runner
- Update .gitignore to allow .gitea/ directory

.gitignore

@@ -10,6 +10,7 @@
 !/.pre-commit-config.yaml
 !/.pylintrc
 !/.github/
+!/.gitea/
 !/contrib/.vscode/
 !*.gitattributes
 !*.gitignore