refactor: split store.go into focused files by responsibility (task o6)

Split 562-line store.go into four files:
- store.go (264 lines): Store type, WalkIndexes, Search, action dispatch
- store_crud.go (160 lines): Add, Import, Remove, buildPair
- store_picker.go (160 lines): Fzf, PickerResult, key parsing
- store_shred.go (54 lines): ShredFile, ShredAllExported

No API or behaviour changes. All functions under 30 lines, all files under 300 lines.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions