chore: move config diagnostics out of codex-core (#12427)

## Why

Compiling `codex-rs/core` is a bottleneck for local iteration, so this
change continues the ongoing extraction of config-related functionality
out of `codex-core` and into `codex-config`.

The goal is not just to move code, but to reduce `codex-core` ownership
and indirection so more code depends on `codex-config` directly.

## What Changed

- Moved config diagnostics logic from
`core/src/config_loader/diagnostics.rs` into
`config/src/diagnostics.rs`.
- Updated `codex-core` to use `codex-config` diagnostics types/functions
directly where possible.
- Removed the `core/src/config_loader/diagnostics.rs` shim module
entirely; the remaining `ConfigToml`-specific calls are in
`core/src/config_loader/mod.rs`.
- Moved `CONFIG_TOML_FILE` into `codex-config` and updated existing
references to use `codex_config::CONFIG_TOML_FILE` directly.
- Added a direct `codex-config` dependency to `codex-cli` for its
`CONFIG_TOML_FILE` use.

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

@@ -1,6 +1,5 @@
 use super::LoaderOverrides;
 use super::load_config_layers_state;
-use crate::config::CONFIG_TOML_FILE;
 use crate::config::ConfigBuilder;
 use crate::config::ConfigOverrides;
 use crate::config::ConfigToml;
@@ -15,6 +14,7 @@ use crate::config_loader::ConfigRequirementsWithSources;
 use crate::config_loader::RequirementSource;
 use crate::config_loader::load_requirements_toml;
 use crate::config_loader::version_for_toml;
+use codex_config::CONFIG_TOML_FILE;
 use codex_protocol::config_types::TrustLevel;
 use codex_protocol::config_types::WebSearchMode;
 use codex_protocol::protocol::AskForApproval;
@@ -153,7 +153,7 @@ async fn returns_config_error_for_schema_error_in_user_config() {
     let config_error = config_error_from_io(&err);
     let _guard = codex_utils_absolute_path::AbsolutePathBufGuard::new(tmp.path());
     let expected_config_error =
-        super::diagnostics::config_error_from_config_toml(&config_path, contents)
+        codex_config::config_error_from_typed_toml::<ConfigToml>(&config_path, contents)
             .expect("schema error");
     assert_eq!(config_error, &expected_config_error);
 }
@@ -166,7 +166,7 @@ fn schema_error_points_to_feature_value() {
     std::fs::write(&config_path, contents).expect("write config");
 
     let _guard = codex_utils_absolute_path::AbsolutePathBufGuard::new(tmp.path());
-    let error = super::diagnostics::config_error_from_config_toml(&config_path, contents)
+    let error = codex_config::config_error_from_typed_toml::<ConfigToml>(&config_path, contents)
         .expect("schema error");
 
     let value_line = contents.lines().nth(1).expect("value line");