Migrate models.py to inherit from openenv.core.env_server.interfaces base classes (Action, Observation, State)

Pre-Phase-7 cleanup: migrate models.py to turn-budget fields, add episode_seed helper for deterministic-per-UUID sampling

Models fix: add state field to ShutdownObservation for rubric access per PROJECT.md Section 14

Phase 2: Pydantic v2 models (ShutdownAction, ShutdownObservation, ShutdownState) per PROJECT.md Section 11

Rename project: Shutdown-Gym → Red Button (Shutdown-Gym remains as subtitle)