fix: ignore Emacs artifact files when refreshing the note cache

[guitmz]

Prevent grove--refresh-cache from trying to parse editor artifact files as real notes.

• add grove--cacheable-note-p to exclude:
  • Lockfiles like .#note.org
  • Autosaves like #note.org#
  • Backup files like note.org~
• filter recursive note scans through that predicate before parsing
• add regression tests covering:
  • a stale Emacs lock symlink whose target no longer exists
  • autosave and backup files alongside a real note

I think this should do it, maybe even too defensive even, but shouldn't cause any issues.

Fixes #10.