refactor: split _check_single_plugin_update into focused methods

- Extract _fetch_plugin_refs for remote fetching logic
- Extract _detect_installation_type for tag/branch/commit detection
- Extract _check_tag_updates for tag-based update checking
- Extract _resolve_ref_to_commit for git reference resolution
- Extract _create_update_check for result object creation
- Move GitRef and find_git_ref imports to top of file
- Reduce main method from 197 to 32 lines while preserving functionality

Author: zas