lore(crimson-cult): tie Sztandar shards to Xillith's halberd; reorganize items/

Xillith Isherwood recently finished reassembling the scattered shards of
the Sztandar Szkarłatnego Kultu (the First Patriarch's original staff)
into his halberd, the "Broń Szkarłatnego Patriarchy" - explaining why
Bezimienny's soul, previously fragmented across the shards, is now mostly
reunified. Threaded through the Sztandar item, Bezimienny, and Xillith
Isherwood pages.

Also folds in a prior file move: items/*.md split into items/artifacts/,
items/armor/, items/weapons/, items/gear/ subfolders, with Przedmioty.md
and the two referencing rulebook hubs updated to match, plus a .gitignore
entry for _books/.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019xsdbKAT27PoAX9SWH3MFK

.gitignore

@@ -7,6 +7,7 @@
 # Vault assets: large binaries managed outside git
 _assets/
 _meta/
+_books/
 
 # AI
 .claude/