Restore human-readable purity summary logs

[LennartvdM]

Summary

• restore the natural language purity summary header and observation lines
• remove the telemetry-style summary builder in favour of readable sentences that highlight status, averages, activities, and worst runs
• keep the detailed per-run logging unchanged while ensuring summary errors are still reported

Testing

• Not run (not requested)

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Codex Task

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Mention checker crashes when run details are missing

If the purity checker crashes before generating per‑run results (e.g. the fallback path in computeBatchPuritySummary sets totalRuns but no runs and didCheckerError = true), this logic only looks at checkerErrorRuns derived from summary.runs. With an empty array the count is zero, so the summary logs All ${totalRuns} runs match raw and share totals. even though statusLabel is ERROR, which misleads operators and hides the crash. The detail text should also consider summary.didCheckerError so that an error is reported instead of claiming all runs matched.

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