Why: trimOldest dropped single messages without tool-pair awareness —
after leading users were evicted it could orphan tool results or leave
dangling calls (acceptance clause 2's product-side gap).
Evidence: go test -count=1 ./... green; TestTrimOldestKeepsToolPairs
(pairing holds across repeated trims), TestTrimOldestDropsLeadingOrphanToolRun,
TestTrimOldestStopsWhenGroupCrossesCurrentTurn, TestPrefixHashStableForIdenticalInput;
native pins carry the spill and cache-contract work (Agent-Native#25).
Impact: trimming now evicts whole tool groups; the window may be
slightly exceeded when a group would cross the current turn — pairing
is never broken. Report.Truncations still records the count.
Refs: Agent-Native#25, Agent-Squad#29, N2 plan WP1
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