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

docs/subsystems/core.md

@@ -419,6 +419,43 @@ async resolve(id?: string): Promise<AgentPreset>
  */
 async mount(agentCtx: Context, id?: string): Promise<AgentPreset>
 
+/**
+ * Join one agent to the SAME standing composition another already runs on.
+ *
+ * This is how a child agent inherits its parent's capabilities. It is a bind,
+ * not a mount: the parent's generation is already composed, so the child gets
+ * that exact instance — the same plugin objects, the same tool registrations,
+ * the same prompt sections. Re-resolving the parent's preset by id instead
+ * would re-read the roster, and a composition file edited since the parent
+ * started would hand the child a DIFFERENT generation than the one its
+ * parent's history was produced under (and a preset deleted since would fail
+ * the child outright while its parent keeps running).
+ *
+ * Synchronous and infallible for that reason, which is what lets a child
+ * creation window use it: the two in-process subagent drivers compose their
+ * children inside a synchronous `setup`.
+ *
+ * A parent that joined no preset — a rosterless deployment — yields no join
+ * and no error: there, the model-facing rows sit in the host composition and
+ * the child already sees them through the global layer.
+ * @param agentCtx - the joining agent's scope context.
+ * @param parentCtx - the scope context of the agent whose composition to join.
+ * @returns the preset id joined, or undefined when the parent joined none.
+ * @throws when `agentCtx` carries no scope, or has already joined a preset.
+ */
+composeFrom(agentCtx: Context, parentCtx: Context): string | undefined
+
+/**
+ * The preset one live agent runs on.
+ *
+ * Read from the live scope chain rather than from the session, so it answers
+ * for an agent whose session has not recorded a preset yet — a child agent
+ * whose durable header is being built from its parent's composition.
+ * @param agentCtx - the agent's scope context.
+ * @returns the preset id, or undefined when the agent joined none.
+ */
+composedPreset(agentCtx: Context): string | undefined
+
 /**
  * Read one preset's composition text.
  * @param id - the preset id.