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/config.rs

@@ -52,6 +52,11 @@ pub struct Config {
     ///
     /// If unset the feature is disabled.
     pub notify: Option<Vec<String>>,
+
+    /// The directory that should be treated as the current working directory
+    /// for the session. All relative paths inside the business-logic layer are
+    /// resolved against this path.
+    pub cwd: PathBuf,
 }
 
 /// Base config deserialized from ~/.codex/config.toml.
@@ -135,6 +140,7 @@ where
 #[derive(Default, Debug, Clone)]
 pub struct ConfigOverrides {
     pub model: Option<String>,
+    pub cwd: Option<PathBuf>,
     pub approval_policy: Option<AskForApproval>,
     pub sandbox_policy: Option<SandboxPolicy>,
     pub disable_response_storage: Option<bool>,
@@ -158,6 +164,7 @@ impl Config {
         // Destructure ConfigOverrides fully to ensure all overrides are applied.
         let ConfigOverrides {
             model,
+            cwd,
             approval_policy,
             sandbox_policy,
             disable_response_storage,
@@ -180,6 +187,23 @@ impl Config {
 
         Self {
             model: model.or(cfg.model).unwrap_or_else(default_model),
+            cwd: cwd.map_or_else(
+                || {
+                    tracing::info!("cwd not set, using current dir");
+                    std::env::current_dir().expect("cannot determine current dir")
+                },
+                |p| {
+                    if p.is_absolute() {
+                        p
+                    } else {
+                        // Resolve relative paths against the current working directory.
+                        tracing::info!("cwd is relative, resolving against current dir");
+                        let mut cwd = std::env::current_dir().expect("cannot determine cwd");
+                        cwd.push(p);
+                        cwd
+                    }
+                },
+            ),
             approval_policy: approval_policy
                 .or(cfg.approval_policy)
                 .unwrap_or_else(AskForApproval::default),