feat(optimize): detect session cost outliers

[ozymandiashh]

Summary

• add an optimize finding for sessions that cost more than 2x their peer-session average in the same project
• ignore tiny absolute-cost outliers to avoid noisy findings
• add focused coverage for minimum samples, exact 2x boundary, absolute-cost filtering, and per-project baselines

Part of #12, item 5b.

Validation

• npx tsc --noEmit
• npx vitest run tests/optimize.test.ts
• npx vitest run
• npm run build

Pre-commit review

• Argus-style reviewer: PASS
• Gemini 3.1 Pro Preview: PASS