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/core/src/codex_wrapper.rs

@@ -26,6 +26,7 @@ pub async fn init_codex(config: Config) -> anyhow::Result<(CodexWrapper, Event,
             sandbox_policy: config.sandbox_policy,
             disable_response_storage: config.disable_response_storage,
             notify: config.notify.clone(),
+            cwd: config.cwd.clone(),
         })
         .await?;