- Add routing.session-affinity-subagents (defaulting to true) to RoutingConfig. - Unify subagent parent credential inheritance across all providers by removing hardcoded provider/model blacklists. - When session-affinity-subagents is false, isolate subagents and distribute them via the fallback selector. - Ensure changing session-affinity-subagents is a no-op when session-affinity is false. - Preserve alias isolation and failure isolation invariants. Closes: #5417