feat(city): app loop, terminal guard, save format, gen CLI

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/city/src/main.rs

@@ -1,3 +1,14 @@
-fn main() {
-    println!("city");
+//! `city`: generate, explore and walk a cyberpunk city in the terminal.
+
+// Screens, widgets and overlays land task by task; lifted once the app is wired up.
+#![allow(dead_code)]
+
+mod app;
+mod cli;
+mod save;
+mod screens;
+mod term;
+
+fn main() -> anyhow::Result<()> {
+    cli::run()
 }