feat: add experiment trend analyzer

[EfeDurmaz16]

Summary

• add analyzeExperimentTrend for pass-rate trend analysis across saved experiment results
• parse both numeric pass rates and existing "50%" summary values
• expose trend result types from the package entrypoint

Closes #102.

Verification

• npm ci
• npm run test -w packages/agent-eval -- src/lib/results.test.ts
• npm run build -w packages/agent-eval
• npm run lint -w packages/agent-eval
• git diff --check

Note: npm ci reports existing dependency audit findings, but installation completed and the focused checks passed.