Add codex app macOS launcher (#10418)

- Add `codex app <path>` to launch the Codex Desktop app.
- On macOS, auto-downloads the DMG if missing; non-macOS prints a link
to chatgpt.com/codex.

codex-rs/cli/src/app_cmd.rs

@@ -0,0 +1,21 @@
+use clap::Parser;
+use std::path::PathBuf;
+
+const DEFAULT_CODEX_DMG_URL: &str = "https://persistent.oaistatic.com/codex-app-prod/Codex.dmg";
+
+#[derive(Debug, Parser)]
+pub struct AppCommand {
+    /// Workspace path to open in Codex Desktop.
+    #[arg(value_name = "PATH", default_value = ".")]
+    pub path: PathBuf,
+
+    /// Override the macOS DMG download URL (advanced).
+    #[arg(long, default_value = DEFAULT_CODEX_DMG_URL)]
+    pub download_url: String,
+}
+
+#[cfg(target_os = "macos")]
+pub async fn run_app(cmd: AppCommand) -> anyhow::Result<()> {
+    let workspace = std::fs::canonicalize(&cmd.path).unwrap_or(cmd.path);
+    crate::desktop_app::run_app_open_or_install(workspace, cmd.download_url).await
+}