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/tui/src/cli.rs

@@ -1,4 +1,5 @@
 use clap::Parser;
+use clap::ValueHint;
 use codex_common::ApprovalModeCliArg;
 use codex_common::CliConfigOverrides;
 use std::path::PathBuf;
@@ -72,6 +73,10 @@ pub struct Cli {
     #[arg(long = "search", default_value_t = false)]
     pub web_search: bool,
 
+    /// Additional directories that should be writable alongside the primary workspace.
+    #[arg(long = "add-dir", value_name = "DIR", value_hint = ValueHint::DirPath)]
+    pub add_dir: Vec<PathBuf>,
+
     #[clap(skip)]
     pub config_overrides: CliConfigOverrides,
 }