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/exec/src/lib.rs

@@ -181,6 +181,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         include_view_image_tool: None,
         show_raw_agent_reasoning: oss.then_some(true),
         tools_web_search_request: None,
+        additional_writable_roots: Vec::new(),
     };
     // Parse `-c` overrides.
     let cli_kv_overrides = match config_overrides.parse_overrides() {