Why: F05 - files-presence could promote failures to success, recovery
marked crashed steps completed on files alone or reran them blindly,
and retries minted a fresh ExecutionID each attempt.
Evidence: squad tests pass incl. executor result contract and recovery
tables (verify-u3b.json); (nil,nil), canceled and side-effect-possible
semantics handled; interrupted steps without a trusted terminal now
hold for reconciliation instead of auto-advancing.
Impact: retries reuse the step ExecutionID and shared deadline; no
auto-replay of unverified external actions.
Refs: feature/agent-stack-audit-remediation-b1 U3B
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