fix(subagent): scope drains and soften final flush

docs/core-data-structures/subagent.md

@@ -141,9 +141,9 @@ Follow-up authority comes from an exact live Agent tool context. The authenticat
 
 For both operations the caller signal owns lookup, materialization, and admission only until inbox acceptance. Afterwards the manager owns the Activation independently: later caller cancellation neither cancels the accepted turn nor disposes the child, and the seam exposes no public subagent cancellation or steering operation.
 
-Every Activation owns its `AgentHandle` and an `ownedChildren: Set<SessionId>`; because one Session has at most one live Activation, the child Session id identifies the live child without another runtime-incarnation reference. Starting a child or submitting parent-originated work registers the child in a continuation-managed parent's set before the child can run, and that parent cannot settle while the set is non-empty. A top-level or other non-continuation Agent has no Activation and stays outside the waiting graph. Child release happens only after the child Agent is quiescent, every child of that child is disposed, the final durability checkpoint settles, and the child's `AgentHandle` completes disposal.
+Every Activation owns its `AgentHandle` and an `ownedChildren: Set<SessionId>`; because one Session has at most one live Activation, the child Session id identifies the live child without another runtime-incarnation reference. Starting a child or submitting parent-originated work registers the child in a continuation-managed parent's set before the child can run, and that parent cannot settle while the set is non-empty. A top-level or other non-continuation Agent has no Activation and stays outside the waiting graph. Child release happens only after the child Agent is quiescent, every child of that child is disposed, the best-effort final session flush settles, and the child's `AgentHandle` completes disposal.
 
-Only `ctx.sessions.flush(session) === true` confirms durability; `false` or rejection reports `DURABILITY_FAILED`. Either way the manager still disposes the handle and releases ownership, because retaining a failed child would permanently pin its ancestors in `waiting` — the persisted child state may then be missing or stale on a later resume. Manager unload invokes an internal manager-wide drain that closes admission and disposes every live forest; `drainContinuableDescendants(parents)` closes admission only below exact live host-owned Agents and disposes their continuable descendants while unrelated forests remain live. Both await already-admitted materializations in their scope, propagate cancellation top-down, release handles child-first, and await every selected branch despite individual failures. Durable child Sessions survive that process-local teardown.
+Final settlement awaits `ctx.sessions.flush(session)` but ignores its participation boolean because an arbitrary listener cannot prove that a persistence backend stored the state. Rejection is logged without failing the Activation, and the manager still disposes the handle and releases ownership; the persisted child state may then be missing or stale on a later resume. Manager unload invokes an internal manager-wide drain that closes admission and disposes every live forest; `drainContinuableDescendants(parents)` closes admission only below exact live host-owned Agents and disposes their continuable descendants while unrelated forests remain live. Both await already-admitted materializations in their scope, propagate cancellation top-down, release handles child-first, and await every selected branch despite individual failures. Durable child Sessions survive that process-local teardown.
 
 ```ts type-equiv
 /** Attribution for a model coordinator's follow-up to one of its children. */