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/lib.rs

@@ -22,6 +22,7 @@ pub use auth::AuthDotJson;
 pub use auth::AuthManager;
 pub use auth::AuthManagerConfig;
 pub use auth::CLIENT_ID;
+pub use auth::CODEX_AGENT_IDENTITY_ENV_VAR;
 pub use auth::CODEX_API_KEY_ENV_VAR;
 pub use auth::CodexAuth;
 pub use auth::ExternalAuth;
@@ -37,9 +38,11 @@ pub use auth::UnauthorizedRecovery;
 pub use auth::default_client;
 pub use auth::enforce_login_restrictions;
 pub use auth::load_auth_dot_json;
+pub use auth::login_with_agent_identity;
 pub use auth::login_with_api_key;
 pub use auth::logout;
 pub use auth::logout_with_revoke;
+pub use auth::read_codex_agent_identity_from_env;
 pub use auth::read_openai_api_key_from_env;
 pub use auth::save_auth;
 pub use auth_env_telemetry::AuthEnvTelemetry;