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/config/Cargo.toml

@@ -16,9 +16,13 @@ futures = { workspace = true, features = ["alloc", "std"] }
 multimap = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
+serde_path_to_error = { workspace = true }
 sha2 = { workspace = true }
 thiserror = { workspace = true }
+tokio = { workspace = true, features = ["fs"] }
 toml = { workspace = true }
+toml_edit = { workspace = true }
+tracing = { workspace = true }
 
 [dev-dependencies]
 anyhow = { workspace = true }