fix: Fix CHANGELOG newline formatting in GitHub Action

Replace sed command with heredoc approach to properly format
newlines in CHANGELOG.md. The previous sed command was outputting
literal \n characters instead of actual newlines.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kiyotis

.github/workflows/sync-to-nabledge.yml

@@ -67,7 +67,27 @@ jobs:
           DATE=$(date +%Y-%m-%d)
 
           # Append sync entry to nabledge-6 plugin CHANGELOG
-          sed -i "/^# Changelog/a \\\n## [Unreleased] - ${DATE}\\\n\\\n### Changed\\\n- Synced from: ${TRIGGER_COMMIT_URL}\\\n" nabledge-repo/plugins/nabledge-6/CHANGELOG.md
+          CHANGELOG_FILE="nabledge-repo/plugins/nabledge-6/CHANGELOG.md"
+          TEMP_FILE=$(mktemp)
+
+          # Read first line (# Changelog)
+          head -n 1 "$CHANGELOG_FILE" > "$TEMP_FILE"
+
+          # Add new unreleased section
+          cat >> "$TEMP_FILE" <<EOF
+
+## [Unreleased] - ${DATE}
+
+### Changed
+- Synced from: ${TRIGGER_COMMIT_URL}
+
+EOF
+
+          # Append rest of original file
+          tail -n +2 "$CHANGELOG_FILE" >> "$TEMP_FILE"
+
+          # Replace original file
+          mv "$TEMP_FILE" "$CHANGELOG_FILE"
 
       - name: Validate marketplace structure
         run: |