fix: --system is not a valid Claude Code CLI flag

Removed the --system argument from the PTY runner — Claude Code CLI
doesn't support it. Bot name instruction is now prepended to the user
prompt instead of passed as a system prompt argument.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/matrix/bot.rs

@@ -748,11 +748,10 @@ async fn handle_message(
 
     // The prompt is just the current message with sender attribution.
     // Prior conversation context is carried by the Claude Code session.
-    let prompt = format_user_prompt(&sender, &user_message);
-
     let bot_name = &ctx.bot_name;
-    let system_prompt = format!(
-        "Your name is {bot_name}. Refer to yourself as {bot_name}, not Claude."
+    let prompt = format!(
+        "[Your name is {bot_name}. Refer to yourself as {bot_name}, not Claude.]\n\n{}",
+        format_user_prompt(&sender, &user_message)
     );
 
     let provider = ClaudeCodeProvider::new();
@@ -792,7 +791,7 @@ async fn handle_message(
         &prompt,
         &project_root_str,
         resume_session_id.as_deref(),
-        Some(&system_prompt),
+        None,
         &mut cancel_rx,
         move |token| {
             let mut buf = buffer_for_callback.lock().unwrap();