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.

scripts/gen-cordis-catalog.ts

@@ -180,6 +180,7 @@ export const EVENT_WALK_EXEMPTIONS: Record<string, string> = {
   'credentials/changed': 'client-face registry invalidation signal — packages/client/runtime/README.md owns the surface',
   'locale/change': 'client-face locale switch signal — packages/client/locale/README.md owns the surface',
   'models/changed': 'client-face registry invalidation signal — packages/client/runtime/README.md owns the surface',
+  'session/preset-changed': 'client-face per-session catalog invalidation signal — packages/client/runtime/README.md owns the surface',
   'settings/changed': 'client-face registry invalidation signal — packages/client/runtime/README.md owns the surface',
   'slash/input-begin-command': 'client-face slash-input protocol — packages/client/ui-slash/README.md owns the surface',
   'slash/input-consume-token': 'client-face slash-input protocol — packages/client/ui-slash/README.md owns the surface',