feat: load AgentIdentity from JWT login/env (#18904)

## Summary

This PR lets programmatic AgentIdentity users provide one token through
either stdin login or environment auth.

`codex login --with-agent-identity` reads an Agent Identity JWT from
stdin, validates that it has the required claims, and stores that token
as the `agent_identity` value in `auth.json`. The file format is
token-only; the decoded account and key fields are runtime state, not
hand-authored auth.json fields.

The Agent Identity JWT claim shape and decoder live in
`codex-agent-identity`; `codex-login` only owns env/storage precedence
and conversion into `CodexAuth::AgentIdentity`.

When env auth is enabled, `CODEX_AGENT_IDENTITY` can provide the same
JWT without writing auth state to disk. `CODEX_API_KEY` still wins if
both env vars are set.

Reference old stack: https://github.com/openai/codex/pull/17387/changes
Reference JWT/env stack: https://github.com/openai/codex/pull/18176

## Stack

1. https://github.com/openai/codex/pull/18757: full revert
2. https://github.com/openai/codex/pull/18871: isolated Agent Identity
crate
3. https://github.com/openai/codex/pull/18785: explicit AgentIdentity
auth mode and startup task allocation
4. https://github.com/openai/codex/pull/18811: migrate Codex backend
auth callsites through AuthProvider
5. This PR: accept AgentIdentity JWTs through login/env

## Testing

Tests: targeted login and Agent Identity crate tests, CLI checks, scoped
formatter/linter cleanup, and CI.

---------

Co-authored-by: Shijie Rao <shijie.rao@openai.com>

codex-rs/login/src/auth/storage.rs

@@ -19,6 +19,7 @@ use std::sync::Mutex;
 use tracing::warn;
 
 use crate::token_data::TokenData;
+use codex_agent_identity::decode_agent_identity_jwt;
 use codex_app_server_protocol::AuthMode;
 use codex_config::types::AuthCredentialsStoreMode;
 use codex_keyring_store::DefaultKeyringStore;
@@ -42,7 +43,7 @@ pub struct AuthDotJson {
     pub last_refresh: Option<DateTime<Utc>>,
 
     #[serde(default, skip_serializing_if = "Option::is_none")]
-    pub agent_identity: Option<AgentIdentityAuthRecord>,
+    pub agent_identity: Option<String>,
 }
 
 #[derive(Deserialize, Serialize, Clone, Debug, PartialEq, Eq)]
@@ -56,6 +57,23 @@ pub struct AgentIdentityAuthRecord {
     pub chatgpt_account_is_fedramp: bool,
 }
 
+impl AgentIdentityAuthRecord {
+    pub(crate) fn from_agent_identity_jwt(jwt: &str) -> std::io::Result<Self> {
+        let claims = decode_agent_identity_jwt(jwt, /*public_key_base64*/ None)
+            .map_err(std::io::Error::other)?;
+
+        Ok(Self {
+            agent_runtime_id: claims.agent_runtime_id,
+            agent_private_key: claims.agent_private_key,
+            account_id: claims.account_id,
+            chatgpt_user_id: claims.chatgpt_user_id,
+            email: claims.email,
+            plan_type: claims.plan_type,
+            chatgpt_account_is_fedramp: claims.chatgpt_account_is_fedramp,
+        })
+    }
+}
+
 pub(super) fn get_auth_file(codex_home: &Path) -> PathBuf {
     codex_home.join("auth.json")
 }