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/tests/rpc-schemas.spec.ts

@@ -32,6 +32,7 @@ import {
   commandListRequestSchema, commandListValueSchema,
 } from '../src/api/commands.schema.ts'
 import { skillEntrySchema, skillListRequestSchema, skillListValueSchema } from '../src/api/skills.schema.ts'
+import { agentPresetEntrySchema, agentPresetListValueSchema } from '../src/api/agent-presets.schema.ts'
 import { hostFrameSchema, muxFrameSchema, askUserQuestionItemSchema } from '../src/api/events.schema.ts'
 import { approvalRequestIdSchema, approvalResponsePayloadSchema } from '../src/api/approvals.schema.ts'
 import { askUserQuestionAnswerSchema, questionResponsePayloadSchema } from '../src/api/questions.schema.ts'
@@ -502,3 +503,16 @@ describe('respond payload schemas', () => {
     expect(payload.sessionId).toBe('s')
   })
 })
+
+describe('agent-preset schemas', () => {
+  it('accepts a roster row and rejects an unknown trust', () => {
+    expect(agentPresetEntrySchema.parse({ id: 'standard', trust: 'system', isDefault: true }))
+      .toEqual({ id: 'standard', trust: 'system', isDefault: true })
+    expect(() => agentPresetEntrySchema.parse({ id: 'x', trust: 'root', isDefault: false })).toThrow()
+    expect(() => agentPresetEntrySchema.parse({ id: '', trust: 'user', isDefault: false })).toThrow()
+  })
+
+  it('accepts an empty roster', () => {
+    expect(agentPresetListValueSchema.parse({ presets: [] })).toEqual({ presets: [] })
+  })
+})