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/host/apiproxy/src/api/sessions.schema.ts

@@ -55,6 +55,7 @@ export const sessionSummarySchema = z.object({
   parentSessionId: sessionIdSchema.optional(),
   origin: z.literal('subagent').optional(),
   cwd: z.string().optional(),
+  agentPreset: z.string().optional(),
   projections: z.lazy(() => sessionProjectionsBlockSchema).optional(),
 }) as unknown as z.ZodType<Wire<SessionSummary>>