Add simple Kakao schedule actions and briefings

[fakeminjun7321]

Summary

• add confirmation-gated personal event create, reschedule, complete, and delete commands
• add compact today briefing and subject/teacher/recent file search
• keep five simple quick replies and concise help
• add schema v6 short-lived pending action state
• clear competing file/event confirmations so only the latest action can execute

Safety

• only registered users can write
• Kakao writes are always scoped to the requester's personal events
• class-wide and other members' events/files remain inaccessible
• every mutation requires confirmation and expires after 10 minutes

Verification

• apps/classbot: npm run check (112/112 tests + Vite build)
• syntax checks and git diff --check