Use AI-generated summary as PR commit message instead of hardcoded string

Co-authored-by: Florian Rival <4ian@users.noreply.github.com>

Author: cursoragent

.github/workflows/improve-docs.yml

@@ -76,7 +76,7 @@ jobs:
         uses: peter-evans/create-pull-request@v7
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
-          commit-message: "docs: AI-driven documentation improvement"
+          commit-message: ${{ steps.improve.outputs.summary }}
           title: "Auto: Improve documentation"
           body: |
             This PR was automatically generated by the **Improve documentation** workflow.