cleanup: remove agent->garden compat shims and narrow garden:register to create-only
- Remove migrate_agent_sid/1 from Garden.Storage (all gardens are on 0.8.0+)
- Narrow garden:register permission from all(Garden) to create(Garden)
- Update socket auth check from read? to create? to match narrowed permission
sow-132
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>