chore: enusre the logic that creates ConfigLayerStack has access to cwd (#8353)

`load_config_layers_state()` should load config from a
`.codex/config.toml` in any folder between the `cwd` for a thread and
the project root. Though in order to do that,
`load_config_layers_state()` needs to know what the `cwd` is, so this PR
does the work to thread the `cwd` through for existing callsites.

A notable exception is the `/config` endpoint in app server for which a
`cwd` is not guaranteed to be associated with the query, so the `cwd`
param is `Option<AbsolutePathBuf>` to account for this case.

The logic to make use of the `cwd` will be done in a follow-up PR.

codex-rs/core/src/config_loader/README.md

@@ -10,7 +10,7 @@ This module is the canonical place to **load and describe Codex configuration la
 
 Exported from `codex_core::config_loader`:
 
-- `load_config_layers_state(codex_home, cli_overrides, overrides) -> ConfigLayerStack`
+- `load_config_layers_state(codex_home, cwd_opt, cli_overrides, overrides) -> ConfigLayerStack`
 - `ConfigLayerStack`
   - `effective_config() -> toml::Value`
   - `origins() -> HashMap<String, ConfigLayerMetadata>`
@@ -37,11 +37,14 @@ Most callers want the effective config plus metadata:
 
 ```rust
 use codex_core::config_loader::{load_config_layers_state, LoaderOverrides};
+use codex_utils_absolute_path::AbsolutePathBuf;
 use toml::Value as TomlValue;
 
 let cli_overrides: Vec<(String, TomlValue)> = Vec::new();
+let cwd = AbsolutePathBuf::current_dir()?;
 let layers = load_config_layers_state(
     &codex_home,
+    Some(cwd),
     &cli_overrides,
     LoaderOverrides::default(),
 ).await?;

codex-rs/core/src/config_loader/mod.rs

@@ -55,8 +55,14 @@ const DEFAULT_REQUIREMENTS_TOML_FILE_UNIX: &str = "/etc/codex/requirements.toml"
 /// (*) Only available on macOS via managed device profiles.
 ///
 /// See https://developers.openai.com/codex/security for details.
+///
+/// When loading the config stack for a thread, there should be a `cwd`
+/// associated with it such that `cwd` should be `Some(...)`. Only for
+/// thread-agnostic config loading (e.g., for the app server's `/config`
+/// endpoint) should `cwd` be `None`.
 pub async fn load_config_layers_state(
     codex_home: &Path,
+    cwd: Option<AbsolutePathBuf>,
     cli_overrides: &[(String, TomlValue)],
     overrides: LoaderOverrides,
 ) -> io::Result<ConfigLayerStack> {
@@ -122,6 +128,7 @@ pub async fn load_config_layers_state(
     }
 
     // TODO(mbolin): Add layers for cwd, tree, and repo config files.
+    let _ = cwd;
 
     // Add a layer for runtime overrides from the CLI or UI, if any exist.
     if !cli_overrides.is_empty() {

codex-rs/core/src/config_loader/tests.rs

@@ -5,6 +5,7 @@ use crate::config_loader::ConfigRequirements;
 use crate::config_loader::config_requirements::ConfigRequirementsToml;
 use crate::config_loader::load_requirements_toml;
 use codex_protocol::protocol::AskForApproval;
+use codex_utils_absolute_path::AbsolutePathBuf;
 use pretty_assertions::assert_eq;
 use tempfile::tempdir;
 use toml::Value as TomlValue;
@@ -40,9 +41,15 @@ extra = true
         managed_preferences_base64: None,
     };
 
-    let state = load_config_layers_state(tmp.path(), &[] as &[(String, TomlValue)], overrides)
-        .await
-        .expect("load config");
+    let cwd = AbsolutePathBuf::try_from(tmp.path()).expect("cwd");
+    let state = load_config_layers_state(
+        tmp.path(),
+        Some(cwd),
+        &[] as &[(String, TomlValue)],
+        overrides,
+    )
+    .await
+    .expect("load config");
     let loaded = state.effective_config();
     let table = loaded.as_table().expect("top-level table expected");
 
@@ -68,9 +75,15 @@ async fn returns_empty_when_all_layers_missing() {
         managed_preferences_base64: None,
     };
 
-    let layers = load_config_layers_state(tmp.path(), &[] as &[(String, TomlValue)], overrides)
-        .await
-        .expect("load layers");
+    let cwd = AbsolutePathBuf::try_from(tmp.path()).expect("cwd");
+    let layers = load_config_layers_state(
+        tmp.path(),
+        Some(cwd),
+        &[] as &[(String, TomlValue)],
+        overrides,
+    )
+    .await
+    .expect("load layers");
     assert!(
         layers.get_user_layer().is_none(),
         "no user layer when CODEX_HOME/config.toml does not exist"
@@ -138,9 +151,15 @@ flag = true
         managed_preferences_base64: Some(encoded),
     };
 
-    let state = load_config_layers_state(tmp.path(), &[] as &[(String, TomlValue)], overrides)
-        .await
-        .expect("load config");
+    let cwd = AbsolutePathBuf::try_from(tmp.path()).expect("cwd");
+    let state = load_config_layers_state(
+        tmp.path(),
+        Some(cwd),
+        &[] as &[(String, TomlValue)],
+        overrides,
+    )
+    .await
+    .expect("load config");
     let loaded = state.effective_config();
     let nested = loaded
         .get("nested")