ci: derive topic labels from PR title (#38595)

Derives topic labels from the PR title scope: `enhance(actions): ...`
gets `topic/gitea-actions`. Scopes map to `topic/<scope>`, with aliases
for names that differ (`oauth` → `topic/authentication`).

Labels stay in sync with the title, while ones applied by hand are left
alone. Scopes with no matching label on the repo are ignored, so no new
labels get created.

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Co-authored-by: Giteabot <teabot@gitea.io>

.github/workflows/pull-labeler.yml

@@ -43,5 +43,6 @@ jobs:
       - run: node ./tools/ci-tools.ts set-pr-labels
         env:
           PR_TITLE: ${{ github.event.pull_request.title }}
+          PR_TITLE_BEFORE: ${{ github.event.changes.title.from }}
           PR_NUMBER: ${{ github.event.pull_request.number }}
           GITHUB_TOKEN: ${{ github.token }}