feat: add --add-dir flag for extra writable roots (#5335)

Add a `--add-dir` CLI flag so sessions can use extra writable roots in
addition to the ones specified in the config file. These are ephemerally
added during the session only.

Fixes #3303
Fixes #2797

codex-rs/app-server/src/codex_message_processor.rs

@@ -1353,6 +1353,7 @@ async fn derive_config_from_params(
         include_view_image_tool: None,
         show_raw_agent_reasoning: None,
         tools_web_search_request: None,
+        additional_writable_roots: Vec::new(),
     };
 
     let cli_overrides = cli_overrides