chore: add cargo fmt check to Rust CI and fix existing violations (#620)

TypeScript CI has prettier --check but Rust CI only runs cargo test.
Add cargo fmt --check to catch formatting issues early, and fix the
8 pre-existing formatting violations on main.

cli/src/connection.rs

@@ -533,15 +533,10 @@ pub fn ensure_daemon(session: &str, opts: &DaemonOptions) -> Result<DaemonResult
     #[cfg(unix)]
     let endpoint_info = format!(
         "socket: {}",
-        get_socket_dir()
-            .join(format!("{}.sock", session))
-            .display()
+        get_socket_dir().join(format!("{}.sock", session)).display()
     );
     #[cfg(windows)]
-    let endpoint_info = format!(
-        "port: 127.0.0.1:{}",
-        get_port_for_session(session)
-    );
+    let endpoint_info = format!("port: 127.0.0.1:{}", get_port_for_session(session));
 
     Err(format!("Daemon failed to start ({})", endpoint_info))
 }