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/codex.rs

@@ -120,7 +120,6 @@ use crate::client_common::Prompt;
 use crate::client_common::ResponseEvent;
 use crate::codex_thread::ThreadConfigSnapshot;
 use crate::compact::collect_user_messages;
-use crate::config::CONFIG_TOML_FILE;
 use crate::config::Config;
 use crate::config::Constrained;
 use crate::config::ConstraintResult;
@@ -136,6 +135,7 @@ use crate::error::CodexErr;
 use crate::error::Result as CodexResult;
 #[cfg(test)]
 use crate::exec::StreamOutput;
+use codex_config::CONFIG_TOML_FILE;
 
 #[derive(Debug, PartialEq)]
 pub enum SteerInputError {