fix(desktop): close stdin for install subprocesses to prevent interactive hangs

Install commands like the Codex CLI script prompt "Start Codex now?
[y/N]" which blocks forever when the subprocess inherits stdin from
the Tauri process. Redirect stdin from /dev/null so any interactive
prompt gets an immediate EOF and defaults to its non-interactive
behavior.

desktop/src-tauri/src/commands/agent_discovery.rs

@@ -129,6 +129,7 @@ fn run_install_command(step: &str, command: &str) -> InstallStepResult {
     }
 
     let mut child = match cmd
+        .stdin(std::process::Stdio::null())
         .stdout(std::process::Stdio::piped())
         .stderr(std::process::Stdio::piped())
         .spawn()