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/exec/src/lib.rs

@@ -29,6 +29,7 @@ pub async fn run_main(cli: Cli) -> anyhow::Result<()> {
         model,
         full_auto,
         sandbox,
+        cwd,
         skip_git_repo_check,
         disable_response_storage,
         color,
@@ -81,6 +82,7 @@ pub async fn run_main(cli: Cli) -> anyhow::Result<()> {
         } else {
             None
         },
+        cwd: cwd.map(|p| p.canonicalize().unwrap_or(p)),
     };
     let config = Config::load_with_overrides(overrides)?;
     let (codex_wrapper, event, ctrl_c) = codex_wrapper::init_codex(config).await?;