Remove: ReplayBuffer, Transition, NStepEntry, push_to_replay,
sample_and_gather, n_step_buffer, replay.rs, --diag-every flag.
PER call sites stubbed with todo!() — replaced by GPU PER in next commits.
Also fixes pre-commit hook to allow todo!() macro (per CLAUDE.md:
"todo!() macro is OK for runtime stubs") while still blocking
TODO/FIXME comment markers.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>