Change BookShelves.Shelf to Return (*Shelf, bool) and Remove StructIsEmpty

[polymorcodeus]

Summary

Changes the BookShelves.Shelf lookup API from a zero-value sentinel to an explicit (*Shelf, bool) return, deletes the reflection-based StructIsEmpty helper, and updates every caller across cmd/book and internal/model. Also fixes the delete completed view and scrubs the legacy polybook name from CONTRIBUTING.md.

What's new

• Explicit shelf lookup API - BookShelves.Shelf(name) now returns (*Shelf, bool), matching the existing Shelf.Collection nil-on-miss pattern.
• Removal of StructIsEmpty - deletes the reflection helper and its test; callers now rely on the boolean or nil checks.
• TUI selection flow fix - markRootScreen passes nil for get/edit/delete, and GetMarkForm shows/hides the mark picker by action, preventing the nil-mark panic in interactive flows.
• Delete success view - ResultView now renders the removed mark instead of the whole collection.
• Docs cleanup - replaces polybook with book in CONTRIBUTING.md.

Behavior changes / guardrails

• A missing shelf now returns (nil, false) instead of &Shelf{}; all production callers have been updated to check the boolean.
• The mark picker is shown for book mark get/edit/remove and hidden for add/list.
• markRootScreen no longer materializes an empty book.Mark; it passes the provided pointer through.
• Delete success output now displays the mark title, URL, and tags rather than the collection's mark list.

Testing

• Added TestBookShelvesShelf covering found and missing shelf lookups.
• Updated cmd/book/actions_test.go to use a testShelf helper that asserts the ok return.
• make check runs fmt, vet, lint, and tests; all green.