fix(results): discover nested eval result dirs

[EfeDurmaz16]

Summary

• Discover eval result directories by walking to directories that contain summary.json instead of only reading the first level under each timestamp.
• Preserve nested eval names such as ui-development/design-system/layout-props as reusable result keys.
• Update housekeeping to operate on nested eval result leaves without deleting the parent subtree as an incomplete result.
• Add regression coverage for nested reusable results and nested housekeeping dedupe.

Fixes #129.

Verification

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