Keep capped Pi reviewer logs parseable as JSONL

[zzwong]

Problem

When Pi reviewer logs reach their size cap, truncation can split a JSONL record. benchmark.ExtractRunMetrics then treats the partial line as malformed JSON and cannot extract the associated metrics.

Impact

Bounded-log runs can lose review telemetry even though the underlying run completed.

Acceptance criteria

• Capped Pi logs retain only complete newline-delimited records before the cap marker, or metrics extraction explicitly tolerates the documented cap marker and partial final record.
• Add regression coverage for metrics extraction from a capped-log fixture.
• Preserve the existing log-size bound.

Evidence: PR #540 review at internal/llmadapters/pi_rpc.go.