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

apps/web/tests/snapshots/subagent-conversation/ui.expected.md

@@ -6,6 +6,8 @@
   - button "1 subagent":
     - text: 1 subagent
     - img
+  - img
+  - text: 标准模式
   - tablist:
     - tab "Chat" [selected]
     - tab "Trajectory"

apps/web/tests/snapshots/subagent-interrupt/offline-composer.expected.md

@@ -3,6 +3,8 @@
     - button "Ask a research subagent to"
     - text: /
     - button "event-sourcing researcher" [disabled]
+  - img
+  - text: 标准模式
   - tablist:
     - tab "Chat" [selected]
     - tab "Trajectory"