Why: the completion doc's N2 requires old tool results to become file paths (ZCode compact pattern) and cache-friendly prefix rules; today the micro-compactor only left a transcript pointer and nothing pinned prefix stability. alignToolTail had no direct edge tests. Evidence: GOWORK=off go vet + go test -count=1 ./... green; new TestClearOldToolResultsSpillsToDisk / TestSpillResultSanitizesCallID / TestAlignToolTailEdges / TestSystemTextDeterministicAcrossIdenticalBuilds; legacy placeholder bytes unchanged when ResultsDir is empty (TestClearOldToolResultsEmptyDirKeepsLegacyPlaceholder). Impact: opt-in via Config.ResultsDir; the native adapter wires it to <workDir>/.agentx/results. No behavior change for other hosts. Refs: 2026-10-07-agent-stack-n2-plan WP1