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/tests/live_agent.rs

@@ -58,6 +58,7 @@ async fn spawn_codex() -> Codex {
                 sandbox_policy: SandboxPolicy::new_read_only_policy(),
                 disable_response_storage: false,
                 notify: None,
+                cwd: std::env::current_dir().unwrap(),
             },
         })
         .await

codex-rs/core/tests/previous_response_id.rs

@@ -98,6 +98,7 @@ async fn keeps_previous_response_id_between_tasks() {
                 sandbox_policy: SandboxPolicy::new_read_only_policy(),
                 disable_response_storage: false,
                 notify: None,
+                cwd: std::env::current_dir().unwrap(),
             },
         })
         .await

codex-rs/core/tests/stream_no_completed.rs

@@ -81,6 +81,7 @@ async fn retries_on_early_close() {
                 sandbox_policy: SandboxPolicy::new_read_only_policy(),
                 disable_response_storage: false,
                 notify: None,
+                cwd: std::env::current_dir().unwrap(),
             },
         })
         .await