fix(apiproxy): echo the preset a created session runs, not its header

`session.create` also adopts an already-live session, and the preceding
commit newly allows adopting one under the preset it switched to while
blank. Its response still echoed `header.agentPreset`, so that adoption
answered with the preset the session had just left — contradicting the
request it had accepted and the row `session.list` serves for the same
session from `resolveSessionPreset()`. The echo now resolves the same way.

The `assertPresetUnchanged` parameter doc said `existing` was the preset
the session was created under; both callers now pass what it runs.

`composeFrom()` was documented as "infallible" and "cannot fail" beside two
`@throws`. It has no composition failure mode — no roster read, no mount, no
file — but it does reject a caller error, and the wording now says which.

The package-level "switched preset" test re-linked to the same preset id,
so it could not tell reading the parent's live scope chain from reading its
creation header. A second fixture preset makes the switch real.

The Web browser lane's subagent goldens gain the preset badge a child now
shows, which is the visible consequence of recording its composition. That
lane runs only under DSH_EXAMPLE_MODE=lib and was missed before.

The Agent Note records two limits found in review: a cold-resumed
continuable child joins its parent's current composition rather than the one
its header names, and `toolFilter` does not constrain a joined child. The
latter is a regression from the agent-plane move rather than anything this
change introduces — with the same tools in the global layer the filter
applies normally — and is tracked in #2185.

Refs #2185

packages/self-modification/tool-cordis/src/api-catalog.ts

@@ -112,7 +112,7 @@ export const SERVICE_API: readonly ServiceApiEntry[] = [
       },
       {
         signature: 'composeFrom(agentCtx: Context, parentCtx: Context): string | undefined',
-        jsDoc: '/**\n * Join one agent to the SAME standing composition another already runs on.\n *\n * This is how a child agent inherits its parent\'s capabilities. It is a bind,\n * not a mount: the parent\'s generation is already composed, so the child gets\n * that exact instance — the same plugin objects, the same tool registrations,\n * the same prompt sections. Re-resolving the parent\'s preset by id instead\n * would re-read the roster, and a composition file edited since the parent\n * started would hand the child a DIFFERENT generation than the one its\n * parent\'s history was produced under (and a preset deleted since would fail\n * the child outright while its parent keeps running).\n *\n * Synchronous and infallible for that reason, which is what lets a child\n * creation window use it: the two in-process subagent drivers compose their\n * children inside a synchronous `setup`.\n *\n * A parent that joined no preset — a rosterless deployment — yields no join\n * and no error: there, the model-facing rows sit in the host composition and\n * the child already sees them through the global layer.\n * @param agentCtx - the joining agent\'s scope context.\n * @param parentCtx - the scope context of the agent whose composition to join.\n * @returns the preset id joined, or undefined when the parent joined none.\n * @throws when `agentCtx` carries no scope, or has already joined a preset.\n */',
+        jsDoc: '/**\n * Join one agent to the SAME standing composition another already runs on.\n *\n * This is how a child agent inherits its parent\'s capabilities. It is a bind,\n * not a mount: the parent\'s generation is already composed, so the child gets\n * that exact instance — the same plugin objects, the same tool registrations,\n * the same prompt sections. Re-resolving the parent\'s preset by id instead\n * would re-read the roster, and a composition file edited since the parent\n * started would hand the child a DIFFERENT generation than the one its\n * parent\'s history was produced under (and a preset deleted since would fail\n * the child outright while its parent keeps running).\n *\n * Synchronous, and with no composition failure mode of its own — it reads no\n * roster, mounts nothing, and touches no file — which is what lets a child\n * creation window use it: the two in-process subagent drivers compose their\n * children inside a synchronous `setup`. It still rejects a caller error, as\n * the `@throws` below record.\n *\n * A parent that joined no preset — a rosterless deployment — yields no join\n * and no error: there, the model-facing rows sit in the host composition and\n * the child already sees them through the global layer.\n * @param agentCtx - the joining agent\'s scope context.\n * @param parentCtx - the scope context of the agent whose composition to join.\n * @returns the preset id joined, or undefined when the parent joined none.\n * @throws when `agentCtx` carries no scope, or has already joined a preset.\n */',
       },
       {
         signature: 'composedPreset(agentCtx: Context): string | undefined',