Move global keyboard shortcut handling from component-level hook to
SkillSaveProvider, eliminating duplicate event listener registrations
and race conditions. Delete use-skill-file-save hook as its logic is
now consolidated in the provider with direct store access.
- use cleanup-based save on tab switch with stable fallback snapshots
- add fallback registry for metadata-only autosave consistency
- add autosave/save-manager tests
Centralize file save operations using Context/Provider pattern for better
maintainability. Add auto-save on tab switch, visibility change, page unload,
and component unmount.
