#215 Replay: Add runtime determinism enforcement

## Problem Determinism relies on developer discipline with no enforcement: - Must use ctx.now instead of datetime.now() - Must use ctx.get_random() + ctx.step() combination - Must wrap all external calls in ctx.step() ## Impact - Silent non-determinism causes divergence during replay - No compile-time or runtime detection - Production bugs invisible in replays ## Current Gaps - No linter to catch datetime.now() usage - No runtime check if random seeded correctly - No warning if get_random() called outside step() ## Solution 1. Add runtime check in DurableContext that warns on datetime.now() detection 2. Create ruff custom rule for determinism violations 3. Add ctx.get_random() validation outside step() context 4. Document all determinism requirements clearly ## Effort: Medium