When given an identifier, explain_decision_cmd now tries it as a
decision_id first (preserving backward compatibility) and, if not found,
falls back to treating it as a plan_id—explaining the root decision of
that plan. The parameter is renamed from decision_id to identifier and
the help text updated to reflect dual usage. The existing BDD step mock
for the "not found" path is updated to also stub list_decisions so the
fallback resolves correctly.
ISSUES CLOSED: #968
brent.edwards