Why: Q1 WP3 - each step execution carries a stable attempt id
(runID/stepID/N) for audit and idempotency, and auto-mode
interrupted recovery must not re-execute a step whose products all
landed before the crash (side effects exactly once).
Evidence: go vet clean; full squad suite ok, including the new
kill-at-superstep-boundary test (completed steps never re-execute,
running-with-products judged complete by attempt) and
missing-product retry tests.
Impact: StepSpec gains AttemptID set when entering running and used
as the execution id; recoverInterrupted (HitlAuto) checks the
step's targets first - all present means completed with artifact
merge, otherwise the legacy pending retry. Transient retries share
the same AttemptID.
Refs: Documents proposal 2026-10-07-agent-stack-n1q1i1-plans.md Q1 WP3
