feat: add last message preview to session picker

Read the final user/assistant message from session transcripts and display
it in the picker alongside the session update time. Allows quick previews
of what's in each session without opening it.

src/gateway/protocol/schema/sessions.ts

@@ -9,6 +9,7 @@ export const SessionsListParamsSchema = Type.Object(
     includeGlobal: Type.Optional(Type.Boolean()),
     includeUnknown: Type.Optional(Type.Boolean()),
     includeDerivedTitles: Type.Optional(Type.Boolean()),
+    includeLastMessage: Type.Optional(Type.Boolean()),
     label: Type.Optional(SessionLabelString),
     spawnedBy: Type.Optional(NonEmptyString),
     agentId: Type.Optional(NonEmptyString),