ci: Add GitHub Actions workflow for marketplace validation

- Validate marketplace.json schema and required fields
- Check for missing plugin.json files in all plugin directories
- Validate plugin.json content and structure
- Check for sensitive information (tokens, API keys)
- Validate package.json npm configuration
- Verify required documentation files exist
- Run Python linting with Black and Pylint
- Execute Python test suite
- Generate validation reports as artifacts
- Trigger on push/PR to main, develop, first_commit branches
- Monitor changes to .claude-plugin and plugin configuration files

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

Author: databricks-platform-bot