feat(web): choose the default agent preset from General settings

One settings row picking which preset new sessions are composed from.

It is deliberately a new-session preference, not a live switch: a session's
preset is fixed at creation and the host refuses to adopt an existing session
under a different one, so the row says "applies to sessions you start from now
on" rather than implying it can retune a running agent.

Options and the current value come from one `agentPreset.list` call — the
roster already reports which id an unspecified session gets, so the row needs
no settings-schema introspection, unlike the permission row it is modelled on.
The write targets only the namespace's `default` field.

The menu marks `user` rows: a locally authored preset is exactly as privileged
as the plugins it names, and presenting it identically to a shipped one would
hide that.

An empty roster reads as `unavailable` and renders nothing, because composing
no presets is a valid deployment rather than a failure — distinct from a
roster call that failed, which surfaces its message.

packages/client/ui-agent-preset/tsconfig.json

@@ -0,0 +1,33 @@
+{
+  "extends": "../../../tsconfig.base.client.json",
+  "compilerOptions": {
+    "rootDir": "src",
+    "outDir": "lib/types"
+  },
+  "include": [
+    "src"
+  ],
+  "references": [
+    {
+      "path": "../connection"
+    },
+    {
+      "path": "../locale"
+    },
+    {
+      "path": "../../../vendor/cordis"
+    },
+    {
+      "path": "../runtime"
+    },
+    {
+      "path": "../ui-primitives"
+    },
+    {
+      "path": "../ui-slots"
+    },
+    {
+      "path": "../../support/invariants"
+    }
+  ]
+}