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/cli/src/main.rs

@@ -870,7 +870,7 @@ fn maybe_print_under_development_feature_warning(
         return;
     }
 
-    let config_path = codex_home.join(codex_core::config::CONFIG_TOML_FILE);
+    let config_path = codex_home.join(codex_config::CONFIG_TOML_FILE);
     eprintln!(
         "Under-development features enabled: {feature}. Under-development features are incomplete and may behave unpredictably. To suppress this warning, set `suppress_unstable_features_warning = true` in {}.",
         config_path.display()