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/index.ts

@@ -63,6 +63,7 @@ export class ApiProxyService extends Service implements ApiProxy {
   readonly commands: ApiProxy['commands']
   readonly goals: ApiProxy['goals']
   readonly skills: ApiProxy['skills']
+  readonly agentPresets: ApiProxy['agentPresets']
   readonly settings: ApiProxy['settings']
   readonly credentials: ApiProxy['credentials']
   readonly llm: ApiProxy['llm']
@@ -85,6 +86,7 @@ export class ApiProxyService extends Service implements ApiProxy {
     this.commands = api.commands
     this.goals = api.goals
     this.skills = api.skills
+    this.agentPresets = api.agentPresets
     this.settings = api.settings
     this.credentials = api.credentials
     this.llm = api.llm