chore: move top-level load_auth() to CodexAuth::from_codex_home() (#1966)

There are two valid ways to create an instance of `CodexAuth`:
`from_api_key()` and `from_codex_home()`. Now both are static methods of
`CodexAuth` and are listed first in the implementation.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/1966).
* #1971
* #1970
* __->__ #1966
* #1965
* #1962

codex-rs/core/src/codex_wrapper.rs

@@ -6,7 +6,7 @@ use crate::config::Config;
 use crate::protocol::Event;
 use crate::protocol::EventMsg;
 use crate::util::notify_on_sigint;
-use codex_login::load_auth;
+use codex_login::CodexAuth;
 use tokio::sync::Notify;
 use uuid::Uuid;
 
@@ -26,7 +26,7 @@ pub struct CodexConversation {
 /// that callers can surface the information to the UI.
 pub async fn init_codex(config: Config) -> anyhow::Result<CodexConversation> {
     let ctrl_c = notify_on_sigint();
-    let auth = load_auth(&config.codex_home)?;
+    let auth = CodexAuth::from_codex_home(&config.codex_home)?;
     let CodexSpawnOk {
         codex,
         init_id,