feat: add NixOS assertions for removed/changed 0.8.0 config keys
Fail evaluation if users provide:
- subscriptions as a list (now a map in 0.8.0)
- deployment_profiles (removed, use activation_args/reboot_policy on subscriptions)
- default_deployment_profile (removed)
SOW-129
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>