docs: design spec for categories and tags (#23)

Records the resolved requirements for issue #23: manual category tree
(no rule engine), single category per item with descendant matching,
central tag registry with hashed-then-overridable colours, AND semantics
for multi-tag filtering, and a drawer-plus-chips storefront filter UI.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -9,3 +9,4 @@ coverage/
 playwright-report/
 test-results/
 .env
+.superpowers/