fix(ui-skill): refresh the slash menu after plugin manager changes

The browser skill source cached each Session's catalog until a preset
switch or connection reset, so an open Session kept listing the skills of
a plugin switched off on the Plugins page and missed those of a plugin
switched on. Clear every cached catalog on the forwarded
plugin-manager/changed event, which the Host emits after the operation's
reload.

Fixes #6268

tsconfig.host.json

@@ -161,6 +161,7 @@
     "apps/web/tests/queue-image.e2e.ts",
     "apps/web/tests/skill-invocation-policy.e2e.ts",
     "apps/web/tests/skill-user-invoke.e2e.ts",
+    "apps/web/tests/skill-menu-plugin-change.e2e.ts",
     "apps/web/tests/permission-policy-context.e2e.ts",
     "apps/web/tests/access-confirmation.e2e.ts",
     "apps/web/tests/agent-preset-selection.e2e.ts",