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/host/apiproxy/tests/api-proxy-agent-preset.spec.ts

@@ -199,6 +199,11 @@ describe('session.create with an agent preset', () => {
     // Comparing against the header would invert both answers: the preset the
     // session actually runs would be refused, and the one it left would pass.
     expect(adopted.result.ok).toBe(true)
+    // The echo has to name the same preset the adoption just accepted, or the
+    // client labels the session with one it has already left — and disagrees
+    // with the row `session.list` serves for it.
+    if (!adopted.result.ok) throw new Error('unreachable')
+    expect(adopted.result.value).toMatchObject({ agentPreset: 'minimal' })
     expect(stale.result.ok).toBe(false)
     if (stale.result.ok) throw new Error('unreachable')
     expect(stale.result.error.details).toMatchObject({ existingPreset: 'minimal' })