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') }),

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

@@ -130,6 +130,17 @@ export type HostFrame =
    * background rather than diffing.
    */
   | { type: 'host/commands-changed' }
+  /**
+   * One blank session was recomposed onto another agent preset (the logged
+   * `agent-preset/selected` commit point, read off the session stream). The
+   * registry-wide `host/commands-changed` cannot stand in for it: recomposing
+   * re-parents that agent's scope without registering anything, so a
+   * preset already mounted for another session produces no registry change
+   * at all. Clients refetch the catalogs this session's composition decides
+   * (`command.list`, `skill.list`) for this sessionId alone; the preset id
+   * rides along for surfaces that label the session.
+   */
+  | { type: 'host/session-preset-changed'; sessionId: SessionId; agentPreset: string }
   /**
    * One settings namespace's resolved value changed (`settings/updated`
    * passthrough) — an RPC write, an external `settings.yaml` edit, or a