refactor(rl): delete host-side PER + diag-every band-aid (greenfield prep)

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>

infra/k8s/argo/alpha-rl-template.yaml

@@ -49,8 +49,6 @@ spec:
         value: "32"
       - name: per-capacity
         value: "32768"
-      - name: diag-every
-        value: "100"
       - name: instrument-mode
         value: "all"
       - name: log-every
@@ -174,7 +172,6 @@ spec:
               --seq-len {{workflow.parameters.seq-len}} \
               --n-backtests {{workflow.parameters.n-backtests}} \
               --per-capacity {{workflow.parameters.per-capacity}} \
-              --diag-every {{workflow.parameters.diag-every}} \
               --seed {{workflow.parameters.seed}} \
               --instrument-mode "{{workflow.parameters.instrument-mode}}" \
               --fold-idx {{workflow.parameters.fold-idx}} \