fix(tui): reset scroll state when clearing messages (#504)

When messages are cleared with /clear or similar commands, the scroll
state was not being properly reset. This caused two bugs:

1. The '↓ End' scroll hint would incorrectly appear after clearing
   because chat_scroll_pinned_bottom was not reset to true
2. The TaskRunning hints (Esc to interrupt) could show incorrectly
   because the scroll state was stale

Fixed by setting chat_scroll_pinned_bottom = true in clear_messages()
to ensure the UI correctly reflects that we're at the bottom of the
(now empty) message list.

Author: echobt

cortex-tui/src/app.rs

@@ -890,6 +890,8 @@ impl AppState {
     pub fn clear_messages(&mut self) {
         self.messages.clear();
         self.chat_scroll = 0;
+        // Reset scroll state to bottom (no "↓ End" hint after clearing)
+        self.chat_scroll_pinned_bottom = true;
         // Request clearing terminal scrollback buffer for privacy
         self.pending_scrollback_clear = true;
     }