feat(web): compose a web session's agent from a named preset

`session.create` takes an optional `agentPreset`, and the host resolves it,
mounts it during pre-publication setup, and records the resolved id on the
session header so a later resume rebuilds the same agent.

Resolution happens BEFORE the session exists, not inside setup: the session
boundary snapshots `meta` before asynchronous setup begins, so an id discovered
during setup could never reach the header. Mounting still happens in setup,
where a failure rolls the whole creation back rather than publishing a session
whose capabilities are half-installed.

Resume ignores whatever the request names and rebuilds from the stored id. A
resumed session's history was produced under that composition; restoring a
different one would replay tool calls the model can no longer make.

`dsh-agent-presets` now throws `UnknownPresetError` / `PresetMountError` so
the host can tell a bad request from a broken preset — they become
`agent-preset-not-found` and `agent-preset-invalid`.

Ships the two built-in compositions (`standard`, `core-web`) and the persona
row that lets them differ in identity. Nothing mounts them yet: no roster is
configured, so `composeAgent` finds no service and every session keeps the host
composition. Wiring the roster and moving base's agent-plane rows behind it is
the next commit, so the switch happens atomically with a real-composition test.

packages/host/apiproxy/src/api/rpc.ts

@@ -44,6 +44,8 @@ export interface RpcErrorDetailsMap {
   'directory-exists': { path: string }
   'directory-create-failed': { path: string }
   'directory-picker-unavailable': { capability: string }
+  'agent-preset-not-found': { agentPreset: string; available: string[] }
+  'agent-preset-invalid': { agentPreset: string; reason: string }
   'agent-busy': { reason: string }
   'queue-item-not-found': { itemId: MessageId }
   'steer-unavailable': { itemId: MessageId }