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/skills/loader.rs

@@ -806,7 +806,6 @@ fn extract_frontmatter(contents: &str) -> Option<String> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::config::CONFIG_TOML_FILE;
     use crate::config::ConfigBuilder;
     use crate::config::ConfigOverrides;
     use crate::config::ConfigToml;
@@ -817,6 +816,7 @@ mod tests {
     use crate::config_loader::ConfigLayerStack;
     use crate::config_loader::ConfigRequirements;
     use crate::config_loader::ConfigRequirementsToml;
+    use codex_config::CONFIG_TOML_FILE;
     use codex_protocol::config_types::TrustLevel;
     use codex_protocol::protocol::SkillScope;
     use codex_utils_absolute_path::AbsolutePathBuf;