feat: admin API to assign/remove Instructor and TA for course sections

Fix emoji consistency, add AI timeline note, and issue tracking reference

docs: add sprint status, integration gaps, and missing APIs to developer assignment

docs: comprehensive rewrite of 00-MASTER-INDEX.md with sprint status, integration gaps, and enhanced remaining sprints

align implementation plan docs with DB schema and add DB migration SQL

docs: sync implementation plan to sidebar-only rule and fix office hours roles