Fix detect-plugins output: use compact jq to avoid multi-line GITHUB_OUTPUT

jq -s . pretty-prints the JSON array across multiple lines. GitHub Actions
rejects multi-line values written with the key=value syntax to GITHUB_OUTPUT.
Changed to jq -sc . so the array stays on one line.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Noizefield

.github/workflows/build-pr.yml

@@ -38,7 +38,7 @@ jobs:
           echo "$CHANGED_FILES"
           
           # Extract unique plugin names from changes in plugins/ directory
-          PLUGINS=$(echo "$CHANGED_FILES" | grep -E "^plugins/" | cut -d'/' -f2 | sort -u | jq -R . | jq -s .)
+          PLUGINS=$(echo "$CHANGED_FILES" | grep -E "^plugins/" | cut -d'/' -f2 | sort -u | jq -R . | jq -sc .)
           
           echo "Detected plugins: $PLUGINS"
           echo "plugins=$PLUGINS" >> $GITHUB_OUTPUT