feat(x): auto-name chats from the first user message

A tiny background LLM call names new chats in 3-5 words, replacing the
truncated-first-message placeholder once it lands. Manual renames always
win; failures silently keep the placeholder.

Model resolution (getChatTitleModel): chatTitleModel override in
models.json, else the curated gateway flash-lite when the assistant
default routes through the gateway, else the assistant's own provider -
so signed-in users who switched their default to BYOK get titles on
that provider instead of a dead gateway call.

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

@@ -74,4 +74,5 @@ export const LlmModelConfig = z.object({
   meetingNotesModel: ModelOverride.optional(),
   liveNoteAgentModel: ModelOverride.optional(),
   autoPermissionDecisionModel: ModelOverride.optional(),
+  chatTitleModel: ModelOverride.optional(),
 });