feat(web): pick a session's agent preset from the composer

A seat in the composer tool row, left of the model select, showing the preset
THIS session runs — its own recorded one, not the deployment default, because a
resumed session runs what it was created with. `SessionSummary` carries
`agentPreset` for that, alongside `cwd` and `origin`.

The switch exists only while the conversation has not started. After the first
turn the seat becomes a plain label rather than a disabled menu: a greyed
control reads as "temporarily unavailable", when in fact the choice is gone for
good. The seat never asks in that state, and the host refuses independently
with `agent-preset-locked`, so a stale client cannot slip one past it.

A rejected switch restores the previous value and surfaces the host's message
rather than leaving the seat showing something the session is not running.

packages/client/runtime/tests/fake-api.ts

@@ -204,6 +204,8 @@ export class FakeApiClient implements IApiClient {
 
   readonly agentPresets: IApiClient['agentPresets'] = {
     list: (payload: unknown) => this.record('agentPreset.list', payload, Promise.resolve(ok({ presets: [] }))),
+    select: (payload: { agentPreset: string }) =>
+      this.record('agentPreset.select', payload, Promise.resolve(ok({ agentPreset: payload.agentPreset }))),
   }
 
   readonly skills: IApiClient['skills'] = {