#361 CRITICAL: ctx.now is non-deterministic - breaks replay guarantees

## Issue The ctx.now property in engine/durable_context.py:486-503 returns datetime.now(timezone.utc) directly without checkpointing. The docstring claims 'In replay mode, this returns the simulated time frozen at the original execution start' but the implementation doesn't do this. ## Impact - Workflows using ctx.now for business logic decisions will behave differently during replay - Data corruption or incorrect branching on replay - Violates Temporal's determinism principle ## Location engine/durable_context.py:486-503 ## Suggested Fix Either checkpoint the first call per workflow OR return workflow_start_time for determinism