fix: keep retrieval eval read-only

[cafitac]

Summary

• keep retrieval evaluation runs read-only by suppressing retrieval bookkeeping writes while evaluating fixtures
• add a regression test proving eval does not mutate retrieval_count, reinforcement_count, or last_accessed_at
• document the determinism guarantee and refresh the handoff for the v0.1.31 candidate slice

Test Plan

• uv run pytest tests/test_retrieval_evaluation.py::test_evaluate_retrieval_fixtures_does_not_mutate_retrieval_counters -q
• uv run pytest tests/test_retrieval_evaluation.py tests/test_retrieval_trace.py tests/test_hermes_adapter.py -q
• uv run pytest tests/ -q
• uv run python scripts/check_release_metadata.py
• uv run python scripts/smoke_release_readiness.py
• npm pack --dry-run
• git diff --check
• node --check bin/agent-memory.js
• static diff secret scan: finding_count=0