Add `link_skip` Method to `FileHandler` for Hybrid Linking Behavior

[AntonMFernando-NOAA]

Description

• Introduce a new link_skip action in the FileHandler class, which provides a hybrid behavior between link_req and link_opt. This method warns about missing target files but skips creating dead symlinks, preventing broken links in the filesystem.

Requirements

• Add link_skip method to FileHandler: warn on missing target files but skip creating symlinks (no dead links).
  Integrate with sync() factory; maintain backward compatibility with link_req and link_opt.

Acceptance Criteria (Definition of Done)

• Method added and callable via 'link_skip'.
• Warns and skips on missing files; creates symlinks when files exist.
• Handles directories, removes existing links, passes all tests.
• Docstrings updated; no breaking changes.
  (Optional): Suggest A Solution
• Use existing _link_files with new mode='skip': warn and continue on missing files.
• Add method and update factory/docstrings.

[AntonMFernando-NOAA]

no longer needed.