fix(subagent): compose children from their parent's preset

Tool and prompt-section visibility is inherited along dsh-scope's parent
chain, and an agent's scope key is minted with no parent. Per-session agent
presets moved every model-facing row onto the agent plane and made
AgentPresets.mount() the one thing that binds that link, from the api-proxy's
session create, resume, and fork paths. The two in-process subagent drivers
installed only the per-child persona and tool filter, so a child's scope chain
had length one and its registry view resolved the global layer alone — which
is empty wherever a preset roster is composed. One-shot children reached the
model with no tools, continuable ones with only the host-plane `report`, and
neither carried its parent's persona, workspace context, or skill catalog.

AgentPresets.composeFrom() joins one agent to the standing composition another
already runs on. It is a bind, not a mount: the child gets its parent's exact
generation, so a composition edited since the parent started cannot fork it
onto another one, and it is synchronous, which is what lets a child creation
window use it. applyChildComposition() now takes the parent and performs the
join first, making a child composed without it unrepresentable at the call
sites. childSessionMeta() records the joined id so a cold read rebuilds the
composition the child actually ran under.

The audit that followed found two api-proxy readers on the wrong authority:
presenterScopeFor() and the live-agent branch of assertPresetUnchanged() both
read header.agentPreset, which goes stale the moment a blank session switches
preset. A switched session's cold transcript resolved presenters in the older
composition's layer and silently degraded to generic cards, and the gateway
refused to adopt a live session under the preset it actually runs while
accepting the one it left. Both now resolve through resolveSessionPreset(),
matching the resume branch fifteen lines above. The owning architecture Agent
Note carried the stale claim that the header records what a session runs; it
is corrected to name the header/log pair and its three readers.

Fixes #2165

packages/subagent/subagent/README.md

@@ -40,6 +40,10 @@ Start-time features are advertised in `provider.capabilities` because the servic
 - `toolFilter` — apply the requested child tool restriction.
 - `persona` — apply a per-child persona.
 
+Every in-process child is composed by one call, `applyChildComposition(childCtx, parent, composition)`, which joins the parent's agent-preset composition before applying the child's own persona and tool filter. The join is what gives the child its capabilities: with every model-facing row on the agent plane, a child that joined nothing would reach the model with an empty tool registry ([`dsh-agent-presets`](../../preset/agent-presets/README.md)). Taking the parent as a parameter is deliberate — it makes composing a child WITHOUT that join unrepresentable at the call sites, which is the defect the one call exists to prevent. A deployment composing no preset roster joins nothing and needs nothing: its model-facing rows sit in the host composition, where the child already resolves them through the tool registry's global layer.
+
+`childSessionMeta()` records the joined preset id on the child's durable header for the same reason a top-level session records its own: the preset decides the tool schemas and prompt sections the model saw, so a cold read of the child's history has to rebuild that composition rather than the deployment default. It is read from the parent's live scope chain, not from the parent header, because a parent that switched preset while blank runs on the newer composition while its header still names the older one.
+
 Continuable creation is the optional `SubagentProvider.prepareContinuable?()` method: its presence is the capability check, so the service rejects a configured continuable start on a provider without it, while a provider that has it may still serve ordinary one-shot delegations. The method returns only a detached `ContinuableCreateSpec` (`{ seed? }`) — data, never a capability: it carries no Agent, `AgentHandle`, prompt delivery, result, disposal, or resume operation, because the continuation manager owns identity reservation, composition, Agent creation, prompt delivery, cold resume, ownership, and disposal after preparation. A one-shot `SubagentRun` represents one disposable foreground delegation with one result and no cold-resume operation.
 
 ## The durable descriptor