configure per-service model defaults for signed-in users

When signed in, default assistant to gpt-5.4, knowledge graph agents to
gpt-5.4-nano, inline task agent to gpt-5.4-mini, and meeting notes to
gpt-5.4. Add meetingNotesModel config field. Fix summarize_meeting to
use gateway provider when signed in.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/x/packages/shared/src/models.ts

@@ -12,4 +12,5 @@ export const LlmModelConfig = z.object({
   model: z.string(),
   models: z.array(z.string()).optional(),
   knowledgeGraphModel: z.string().optional(),
+  meetingNotesModel: z.string().optional(),
 });