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

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

core/src/types.rs

@@ -166,6 +166,9 @@ pub struct Session {
     /// User-defined group in the session list.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub folder: Option<String>,
+    /// Instructions the user set for this chat; added to the system prompt of every turn.
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    pub instructions: String,
 }
 
 impl Session {