Why: the pin-documents flow exposed guard divergence — precheck counted
submodule drift while task-start checked porcelain only, letting a task
start despite a BUSY verdict; and PR#8 overcorrected by treating
uninitialized (-) submodules as BUSY when they only block builds, not
writes.
Evidence: bash -n clean; precheck on this checkout prints tree line with
drift/uninit split and READY verdict; negative path exercised by the
real divergence that started this task.
Impact: precheck warns on uninitialized, BUSY only on +/U; task-start now
refuses on porcelain dirt and +/U drift, matching precheck.
Refs: PR#8; policy/tool-adaptation-matrix.md A1
Why: the two 2026-09-30 lesson entries merged into documents main; the
composition should consume them for future retrieval.
Evidence: documents PR#3 merged (2ec3e50); make test on this branch before
merge per the gitlink rule.
Impact: gitlink-only advance; no product code change.
Refs: docs/records/2026-09-30-lesson-*.md
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