fix: Potential fix for code scanning alert no. 13: Workflow does not contain permissions

[teolemon]

Potential fix for https://github.com/openfoodfacts/facets-knowledge-panels/security/code-scanning/13

The best way to fix this problem is to explicitly set the permissions key in the workflow or job definition to restrict the GITHUB_TOKEN to only the minimum necessary privileges. In this case, since the job is only validating PR titles, it likely requires read access to contents and may need write access to pull-requests to potentially comment or mark PRs. Add a permissions block either at the workflow root (global for all jobs) or just under the main job. For clarity, add it inside the main job (line 13), specifying:

permissions:
  contents: read
  pull-requests: write

Edit .github/workflows/semantic-pr.yml by inserting this block with proper indentation directly within the main job, before its steps.

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