fix(cortex-tui): clear messages before loading session to prevent duplicates

Fixes bounty issue #1704

The load_session_at_index() function was not clearing existing messages
before loading session messages, causing duplicate display of the last
assistant message when resuming a session. This follows the same pattern
used in InteractiveAction::ResumeSession which correctly clears messages first.

Author: factorydroid

cortex-tui/src/runner/event_loop.rs

@@ -6024,6 +6024,8 @@ impl EventLoop {
             // Load session from storage
             let session_id_str = session_id.to_string();
             if let Ok(loaded_session) = CortexSession::load(&session_id_str) {
+                // Clear existing messages before loading session messages
+                self.app_state.messages.clear();
                 // Load messages into app state
                 for msg in loaded_session.messages() {
                     // Use cortex_core::widgets::Message constructors for UI display