Instructions per chat (system prompt addition; UI dialog and Telegram /prompt)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/types.ts

@@ -67,6 +67,8 @@ export interface Session {
   pinned?: boolean;
   archived?: boolean;
   folder?: string | null;
+  /** Instructions for this chat (added to the system prompt). */
+  instructions?: string;
 }
 
 export interface QuestionOption {