refactor(features): extract builders and reduce barrel

Move the remaining build*/to*/apply* functions plus the private
buildExplanation helper into workspaceAssistantBuilders.ts.
workspaceAssistant.ts reduces to four export-* lines. Every
consumer's existing import string resolves through the barrel.
Wave 5.3.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: saagar210