Replace recursive directory traversal with first-level-only symlink scanning
to fix severe performance issues on large model collections (220K+ files).
- Rename _scan_directory_links to _scan_first_level_symlinks
- Only scan symlinks directly under each root directory
- Skip traversal of normal subdirectories entirely
- Update tests to reflect first-level behavior
- Add test_deep_symlink_not_scanned to document intentional limitation
Startup time reduced from 15+ minutes to seconds for affected users.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>