Why: evidence rows are written before their containing card commit, so a naive
HEAD-diff would flag every task's evidence as stale and train people to ignore
the warning.
Evidence: bash -n clean; the live task-sync for this task exercises the new
path (warning fires only when a non-.task diff follows an evidence row's HEAD).
Impact: scripts/task.sh cmd_sync now excludes .task from the staleness diff;
AGENTS.md wording aligned (three-output evidence via make evidence).
Refs: task card evidence table; guidelines verification.md tool-written rule.
weichen
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