feat(web): expose the agent-preset roster over the API

`agentPreset.list` gives a browser the deployment's roster so it can offer a
choice when starting a session. Each row carries the id, its `trust`, and
whether it is the current default.

`trust` is on the wire deliberately: a `user` preset is exactly as privileged
as the plugins it names, so a surface that offers one alongside a shipped
preset can say which is which rather than presenting both as vetted.

The domain is read-only. A preset is a composition on disk, so authoring one
is a filesystem act rather than an RPC; and a deployment composing no presets
answers with an empty roster rather than an error, because sharing the host
composition is a valid deployment.

The RPC map made every registration site a type error, so the route, the
response-schema table, the service delegate, and the browser fixture are all
wired rather than only the ones I remembered.

packages/host/apiproxy/src/api/agent-presets.schema.ts

@@ -0,0 +1,25 @@
+/**
+ * agent-presets domain zod schemas (names derived from map keys:
+ * agentPresetListRequestSchema / agentPresetListValueSchema).
+ */
+
+import { z } from 'zod'
+import type { RequestPayload, ResponseValue } from './rpc-map.ts'
+import type { Wire } from './rpc.schema.ts'
+import type { AgentPresetEntry } from './agent-presets.ts'
+
+/** AgentPresetEntry row of agentPreset.list. */
+export const agentPresetEntrySchema = z.object({
+  id: z.string().min(1),
+  trust: z.union([z.literal('system'), z.literal('user')]),
+  isDefault: z.boolean(),
+}) satisfies z.ZodType<Wire<AgentPresetEntry>>
+
+/** agentPreset.list request payload. */
+export const agentPresetListRequestSchema = z.object({
+}) satisfies z.ZodType<Wire<RequestPayload<'agentPreset.list'>>>
+
+/** agentPreset.list response value. */
+export const agentPresetListValueSchema = z.object({
+  presets: z.array(agentPresetEntrySchema),
+}) satisfies z.ZodType<Wire<ResponseValue<'agentPreset.list'>>>