fix(bench): load object-starting RAG JSONL exports

[drewstone]

Summary

• fix RAG shared JSON reader so object-starting JSONL exports parse line-by-line instead of as one JSON object
• add a regression test for newline-delimited object rows

Verification

• pnpm exec tsx --test src/benchmarks/rag-benchmarks.test.mts: 7/7 pass
• pnpm exec tsc --noEmit: pass
• RAGBENCH_DATA_FILE=/tmp/ragbench-hotpotqa-validation-5.jsonl BENCHMARKS=ragbench CELLS=deepseek-v4-flash BACKEND=router N=5 CONCURRENCY=1 TIMEOUT_MS=180000 pnpm run run-benchmarks: executes 5/5 tasks, 0 infra errors
• git diff --check: pass
• mergeability: git merge-tree --write-tree origin/main HEAD exit 0

[tangletools]

✅ Auto-approved drewstone PR — b7fa6ebf

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

_{tangletools · auto-approval · reason: drewstone_author · 2026-07-07T23:24:35Z}