`agentFor` fenced subagent ownership through the attached session store
(`ctx.sessions.get`) and only then returned a live registered agent. A
registered agent whose session is ever absent from the attached store —
an invariant nothing in this package guarantees — would therefore be
handed out through generic Host routing unfenced, bypassing subagent
delivery entirely.
Fence `live.session` directly whenever a live agent exists, and keep the
attached-store check only for the not-live durable classification.
`ensureSession`'s race `.catch` already fences `live.session`; this makes
the fast path the same check instead of an asymmetric weaker one.
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