Kill Log: Fleet Mode receipts

Append-only. Every kept/killed decision with the real number. Sample rows below show the format written by scripts/append_receipt.py.

when (UTC)	task	tried	verdict	why	metric	value
2026-06-02 14:11	add retry to uploader	exponential backoff with jitter	kept	cut timeout errors to zero in a 200-run soak	errors	0
2026-06-05 09:47	speed up PR review	4-way parallel reviewer fan-out	killed	duplicated findings and missed the one real bug a single reviewer caught	unique_findings	3 vs 5
2026-06-08 20:30	nightly backup job	rsync to offsite branch plus 14d local retention	adopted	restore drill recovered full state in under a minute	restore_seconds	52