feat: defer coach selection to ad prompt with two-step UI

refine evaluation and fix CSV download problem

tune performance

fix scripts for new prompts

feat: Add database-backed prompt management system with repository pattern

refactor: Remove studentName/coachName from database, derive from templates

feat: Replace summary banner with student background system message

feat: Add graceful degradation for removed student templates

refactor: Simplify architecture by removing AgentRepository

feat: Complete SEL Chat Coach - Traditional Chinese social-emotional learning platform