Add Auto Review proof metrics fixture

[cbusillo]

Summary

• add an Auto Review proof-metrics parity map for Map Auto Review proof metrics onto Codex review primitives #400, including the current background-review size-limit gap
• expose approvals_reviewer on code exec --json thread.started events so CLI fixtures can validate reviewer routing directly
• add and wire a deterministic no-live-token harness scenario proving approvals_reviewer = "auto_review" is accepted and emitted through the current wire-compatible guardian_subagent spelling

Validation

• python3 -m json.tool tools/code-exec-harness/scenarios/auto-review-config-routing.json >/dev/null
• python3 -m py_compile tools/code-exec-harness/harness.py
• git diff --check
• ./build-fast.sh
• python3 tools/code-exec-harness/harness.py tools/code-exec-harness/scenarios/auto-review-config-routing.json --code-bin /Users/cbusillo/.code/worktrees/code/auto-review-proof-metrics/code-rs/target/dev-fast/code
• CODE_EXEC_HARNESS_BIN=/Users/cbusillo/.code/worktrees/code/auto-review-proof-metrics/code-rs/target/dev-fast/code tools/code-exec-harness/run-deterministic.sh

Notes

• Current broad background Auto Review is not counted as validation for this slice: ledger run 398c40ae-0323-4e2b-9e01-46fbb80e8d01 failed before findings because the diff was 1,479,472 chars against the 120,000-char configured background review limit.

Closes #400.