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/preset/agent-presets/README.md

@@ -14,7 +14,7 @@ Discovery is unmemoized: `list()` and `resolve()` re-read the roots on every cal
 - `ctx.agentPresets.list(): Promise<AgentPreset[]>` Every preset the configured roots currently supply, earlier root winning a duplicate id; broken presets included, each carrying its reason.
 - `ctx.agentPresets.resolve(id?): Promise<AgentPreset>` One preset by id, defaulting to `defaultId`. Throws naming the available ids when no root supplies it. A broken preset resolves — deleting, reading, and reporting one all need the row.
 - `ctx.agentPresets.mount(agentCtx, id?): Promise<AgentPreset>` Compose one agent from a preset — ensure its standing mount (single-flight) and parent the agent's scope key to it — returning the preset for the caller to record. Refuses a broken preset up front with its discovery-reported reason, so every unloadable shape fails the same way before the loader is involved.
-- `ctx.agentPresets.composeFrom(agentCtx, parentCtx): string | undefined` Join one agent to the standing composition another already runs on, returning the preset id joined — `undefined` when the parent joined none, which is the rosterless deployment and not an error. A bind rather than a mount, so it is synchronous and cannot fail.
+- `ctx.agentPresets.composeFrom(agentCtx, parentCtx): string | undefined` Join one agent to the standing composition another already runs on, returning the preset id joined — `undefined` when the parent joined none, which is the rosterless deployment and not an error. A bind rather than a mount, so it is synchronous and has no composition failure mode; it still rejects a caller error (an unscoped context, or an agent that already joined).
 - `ctx.agentPresets.composedPreset(agentCtx): string | undefined` The preset one LIVE agent runs on, read from its scope chain rather than from its session — the only answer available for an agent whose durable header is still being built.
 - `ctx.agentPresets.recompose(agentCtx, id): Promise<AgentPreset>` Re-link one agent to a different preset's standing composition. Valid only while the agent has produced nothing — **the caller owns that check**; the new mount is ensured before the link moves, so a failure leaves the agent as it was. Refuses a broken preset like `mount()`.
 - `ctx.agentPresets.standingKeyFor(id?): Promise<ScopeKey>` The standing scope key a host reader with no agent (a cold transcript read) resolves preset registrations in; ensures the mount without starting an agent, session, or turn. Refuses a broken preset like `mount()`.