fix(web): follow a blank session's preset switch in the slash catalog

Presets own the rows that decide what a session's `/` menu contains, but
both browser catalogs cache per session and had no invalidation edge for a
recompose: `commands/changed` is registry-wide and recomposing registers
nothing, so the menu kept serving the composition the session no longer ran.

The host stream now frames the logged `agent-preset/selected` commit as
`host/session-preset-changed`; the runtime bridges it to the typed
`session/preset-changed` event, `ui-command` soft-refreshes that session's
directory key and `ui-skill` invalidates its catalog entry.

Reaching the host on a second switch was a separate defect: the list-row
identity guard compared every summary field except `agentPreset`, and the
merge keeps the row's `updatedAt`, so a switched row looked unchanged and
served its cached instance forever. The hero chip compares the pick against
that row, so switching back to the creation-time preset sent no RPC at all.

packages/host/apiproxy/src/api/events.schema.ts

@@ -82,6 +82,7 @@ export const hostFrameSchema = z.discriminatedUnion('type', [
   z.object({ type: z.literal('host/workspace-removed'), workspaceId: workspaceIdSchema }),
   z.object({ type: z.literal('host/archived-sessions-changed'), archivedSessionIds: z.array(sessionIdSchema) }),
   z.object({ type: z.literal('host/commands-changed') }),
+  z.object({ type: z.literal('host/session-preset-changed'), sessionId: sessionIdSchema, agentPreset: z.string() }),
   z.object({ type: z.literal('host/settings-changed'), ns: z.string() }),
   z.object({ type: z.literal('host/credentials-changed'), ref: z.string() }),
   z.object({ type: z.literal('host/models-changed') }),