fix: discover nested eval result directories

[EfeDurmaz16]

Summary

Fixes result discovery for nested eval ids such as ui-development/design-system/layout-props.

saveResults already writes nested eval result directories correctly, but housekeeping and reusable-result scanning only looked at immediate children of a timestamp directory. That caused nested results to be treated as incomplete or missed for reuse because summary.json lives deeper in the tree.

Changes

• Add a shared recursive discoverEvalResultDirs helper.
• Use it in housekeep so nested eval result directories are deduped/kept by their full relative eval id.
• Use it in scanReusableResults so nested evals can match fingerprint keys and be reused.
• Add regression coverage for nested housekeeping and reusable-result scanning.
• Add a patch changeset.

Validation

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

Closes #129.