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/subagent/subagent-inprocess/tests/fixtures/presets/reviewing/agent.cordis.yml

@@ -0,0 +1,6 @@
+# A second agent-plane composition, so a switch is a real switch: the tool a
+# joined child sees has to change with it.
+- id: only
+  name: ../../plugins/preset-tool.js
+  config:
+    tool: reviewing_only

packages/subagent/subagent-inprocess/tests/preset-inheritance.spec.ts

@@ -105,12 +105,16 @@ describe('a child agent composed in-process', () => {
 
   it('follows a parent that switched preset while blank', async () => {
     const { ctx, parent } = await setupPresetHost()
-    await ctx.agentPresets.recompose(parent.ctx, 'coding')
+    // A DIFFERENT preset, so the assertion below distinguishes reading the
+    // parent's live scope chain from reading its creation header — re-linking
+    // to the same id would pass either way.
+    await ctx.agentPresets.recompose(parent.ctx, 'reviewing')
 
     const run = await startInProcessRun(spawnRequest(parent), {})
     await run.result
 
-    expect(ctx.tools.schemas(run.localAgent).map(schema => schema.name)).toEqual(['preset_only'])
+    expect(ctx.tools.schemas(run.localAgent).map(schema => schema.name)).toEqual(['reviewing_only'])
+    expect(run.localAgent?.session.header.agentPreset).toBe('reviewing')
     await run.dispose()
   })
 })