feat(clean): remove obsolete VS Code and Cursor extension directories (#910)

VS Code, VS Code Insiders and Cursor write a .obsolete JSON marker in
their extensions root whose keys are stale extension directories left
behind after an update. mo clean never read it, so obsolete extensions
accumulated.

Add clean_editor_obsolete_extensions: parse each editor's .obsolete,
validate every key is a plain direct-child name (rejecting path
traversal), and remove the listed directories via safe_clean.