fix(results): restore index jsonl artifact

[christso]

Summary

• restore index.jsonl as the only canonical run row artifact name
• remove the recent alternate filename compatibility path instead of carrying an alias
• update results readers, docs, and tests around the summary.json aggregate vs index.jsonl row-index distinction
• fix the full CLI package test expectation so it asserts the canonical root index.jsonl is written
• speed up CI Validate Evals by reusing the Build job artifact instead of rebuilding packages in that job
• update artifact upload/download actions to Node 24-based major versions

Validation

• bun run build
• bun test apps/cli/test/commands/eval/artifact-writer.test.ts -t "writes summary, index.jsonl, and per-run artifact files"
• bun --filter agentv test passed locally: 735 tests
• bun scripts/validate-eval-dirs.ts && bun apps/cli/dist/cli.js validate 'examples/features/**/evals/**/*.eval.yaml' 'examples/features/**/*.EVAL.yaml'
• bun test apps/cli/test/commands/eval/result-layout.test.ts apps/cli/test/commands/eval/aggregate.test.ts apps/cli/test/commands/results/export.test.ts apps/cli/test/commands/compare/compare.test.ts apps/cli/test/commands/trend/trend.test.ts apps/cli/test/unit/retry-errors.test.ts packages/core/test/evaluation/results-repo.test.ts
• bun run typecheck
• bun run lint
• bun --filter @agentv/core generate:schema exited 0 with existing recursive-reference warnings; generated output had no relevant artifact-name changes and was not kept
• GitHub Actions CI is green on 1270e997; Validate Evals restored the Build artifact and completed in 18s
• Live eval dogfood attempted with bun apps/cli/src/cli.ts eval examples/features/rubric/evals/dataset.eval.yaml --test-id summary-task; provider failed with pi-ai call failed: Connection error, but the emitted run wrote index.jsonl and no removed-name artifact.

Beads: av-wmw4, av-bcqv, av-iuu9, av-8ry5, av-hb10

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