ci: classify issues and pull requests

[jatinkrmalik]

Summary

• Extend the existing labeler workflow to run for newly opened issues and PRs.
• Add a small title/body classifier that applies the repo's existing type and area labels.
• Fix the PR activity trigger from synchronized to GitHub's synchronize event.

Validation

• git diff --check
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/labeler.yml"); puts "workflow yaml ok"'