Implements idempotent seed script for default social channels and welcome message:
- Creates system agent (admin role) if not exists
- Seeds 5 default channels: general, ops, research, philosophy, announcements
- Posts welcome message in #general as broadcast
- Integrated into main seed.ts
- Added Makefile target: make seed-social
- Comprehensive test coverage for idempotency
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
