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/mod.rs

@@ -117,8 +117,6 @@ pub(crate) const PROJECT_DOC_MAX_BYTES: usize = 32 * 1024; // 32 KiB
 pub(crate) const DEFAULT_AGENT_MAX_THREADS: Option<usize> = Some(6);
 pub(crate) const DEFAULT_AGENT_MAX_DEPTH: i32 = 1;
 
-pub const CONFIG_TOML_FILE: &str = "config.toml";
-
 #[cfg(test)]
 pub(crate) fn test_config() -> Config {
     let codex_home = tempdir().expect("create temp dir");
@@ -2302,6 +2300,7 @@ mod tests {
     use crate::config::types::Notifications;
     use crate::config_loader::RequirementSource;
     use crate::features::Feature;
+    use codex_config::CONFIG_TOML_FILE;
 
     use super::*;
     use core_test_support::test_absolute_path;