fix(ci): show modified files when pre-commit hooks fail (#45)

When a pre-commit hook like markdownlint-fix modifies files, the CI
output only says "files were modified by this hook" without indicating
which files changed. This adds a follow-up step that runs on failure
to display both the list of modified files and the full diff.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: jguice

.github/workflows/ci.yml

@@ -32,3 +32,14 @@ jobs:
 
       - name: Run pre-commit hooks
         run: pre-commit run --all-files
+
+      - name: Show files modified by pre-commit hooks
+        if: failure()
+        run: |
+          echo "::group::Files modified by pre-commit hooks"
+          git diff --name-only
+          echo "::endgroup::"
+          echo ""
+          echo "::group::Full diff of modifications"
+          git diff
+          echo "::endgroup::"