feat: make cwd a required field of Config so we stop assuming std::env::current_dir() in a session (#800)

In order to expose Codex via an MCP server, I realized that we should be
taking `cwd` as a parameter rather than assuming
`std::env::current_dir()` as the `cwd`. Specifically, the user may want
to start a session in a directory other than the one where the MCP
server has been started.

This PR makes `cwd: PathBuf` a required field of `Session` and threads
it all the way through, though I think there is still an issue with not
honoring `workdir` for `apply_patch`, which is something we also had to
fix in the TypeScript version: https://github.com/openai/codex/pull/556.

This also adds `-C`/`--cd` to change the cwd via the command line.

To test, I ran:

```
cargo run --bin codex -- exec -C /tmp 'show the output of ls'
```

and verified it showed the contents of my `/tmp` folder instead of
`$PWD`.

codex-rs/tui/src/cli.rs

@@ -28,6 +28,10 @@ pub struct Cli {
     #[clap(flatten)]
     pub sandbox: SandboxPermissionOption,
 
+    /// Tell the agent to use the specified directory as its working root.
+    #[clap(long = "cd", short = 'C', value_name = "DIR")]
+    pub cwd: Option<PathBuf>,
+
     /// Allow running Codex outside a Git repository.
     #[arg(long = "skip-git-repo-check", default_value_t = false)]
     pub skip_git_repo_check: bool,