dsh-teacher: DSH plugin, turning agents into Socratic teachers

dsh-teacher is a plugin-based teaching tool within the DSH ecosystem, designed to transform traditional "direct instruction" into "Socratic questioning guidance." The plugin loads courses from Markdown question sets and persists data in SQLite. Its core strategy is "ask without answering," guiding user thinking by hiding answer keys while recording "knowledge gaps" in real-time during sessions. It integrates the FSRS-5 spaced repetition algorithm to perform on-demand retesting of overdue knowledge gaps, achieving cross-session long-term memory consolidation. Furthermore, it provides immediate quiz popups (supporting multiple choice and fill-in-the-blank) without requiring LLM involvement, followed by an LLM-based scoring and Socratic review of error points after the quiz. The Web client supports viewing the question bank, the knowledge gap panel, and status. Installation requires DSH rc.6+ and Node >= 22.