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

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

server/src/prompts.rs

@@ -216,6 +216,17 @@ fn instruction_files(app: &App, root: &Path, cwd: &Path) -> Vec<(PathBuf, String
 
 /// The plain chat mode: no coding instructions, project files or memory – only the
 /// date, the model and what the user keeps in their personal memory.
+/// Add the instructions the user set for this chat (they come last, so they win).
+pub fn with_chat_instructions(mut system: String, instructions: &str) -> String {
+    if !instructions.trim().is_empty() {
+        system.push_str(&format!(
+            "\n\n# Instructions for this chat\nThe user set these for this chat. They apply to every message and override the defaults above:\n{}\n",
+            instructions.trim()
+        ));
+    }
+    system
+}
+
 fn chat_prompt(app: &App, agent: &Agent, model: &str, owner: Option<&crate::store::User>) -> String {
     let mut s = agent.prompt.clone();
     s.push_str(&format!("\n\n# Context\n- Date: {}\n- Model: {model}\n", chrono::Local::now().format("%A, %Y-%m-%d")));