feat: clean config loading and config api (#7924)

Check the README of the `config_loader` for details

codex-rs/Cargo.lock

@@ -846,12 +846,10 @@ dependencies = [
  "serde",
  "serde_json",
  "serial_test",
- "sha2",
  "shlex",
  "tempfile",
  "tokio",
  "toml 0.9.5",
- "toml_edit",
  "tracing",
  "tracing-subscriber",
  "uuid",