feat: TUI onboarding (#3398)

Example of how onboarding could look like

codex-rs/tui/src/history_cell.rs

@@ -1052,6 +1052,12 @@ pub(crate) fn new_mcp_tools_output(
     PlainHistoryCell { lines }
 }
 
+pub(crate) fn new_info_event(message: String) -> PlainHistoryCell {
+    let lines: Vec<Line<'static>> =
+        vec![vec![padded_emoji("💾").green(), " ".into(), message.into()].into()];
+    PlainHistoryCell { lines }
+}
+
 pub(crate) fn new_error_event(message: String) -> PlainHistoryCell {
     // Use a hair space (U+200A) to create a subtle, near-invisible separation
     // before the text. VS16 is intentionally omitted to keep spacing tighter