refactor: remove help_topics/ and legacy code (Phase 4)

Delete 25 .txt files, embedded FS, switch statement, showKeywords/
showTypes/showDeleteBehaviors functions, and reservedKeywords/
attributeTypes/deleteBehaviors data. All topics now served from
the syntax feature registry with full alias map for backward compat.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: engalar