Fix music-social issue body formatting by normalizing commit message to first line (#246)

* Initial plan

* fix: keep only first commit line in music update issues

* refactor: reuse default commit message constant

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

.github/workflows/music-social.yml

@@ -77,7 +77,10 @@ jobs:
           script: |
             const sha = process.env.COMMIT_SHA || '';
             const shortSha = sha.slice(0, 7);
-            const commitMsg = process.env.COMMIT_MSG || 'Mise à jour manuelle';
+            const defaultCommitMsg = 'Mise à jour manuelle';
+            const commitMsg = (process.env.COMMIT_MSG || defaultCommitMsg)
+              .split('\n')
+              .find(line => line.trim().length > 0) || defaultCommitMsg;
             const author = process.env.COMMIT_AUTHOR || 'DavidKRK';
             const linesAdded = process.env.LINES_ADDED;
             const linesRemoved = process.env.LINES_REMOVED;