feat: persist trace policy links

[hijzy]

Summary

• persist trace-policy evidence links in storage and expose a repo for historical lookup
• include linked traces and episodes when recomputing L2 policy gain
• add adaptive gain baseline and EMA smoothing config with focused L2 tests

Tests

• npm run lint
• npx vitest run tests/unit/memory/l2/gain.test.ts tests/unit/memory/l2/l2.integration.test.ts tests/unit/memory/l2/subscriber.test.ts