## Why
The external startup/login surface for this auth path should talk about
an access token instead of exposing the internal Agent Identity
terminology. Users should pass `CODEX_ACCESS_TOKEN` or pipe a token into
`codex login --with-access-token`; the old external env/flag spellings
are removed so there is only one supported user-facing path.
## What Changed
- Added `CODEX_ACCESS_TOKEN` as the supported environment variable for
this auth path.
- Added `codex login --with-access-token` as the supported stdin-based
login command.
- Removed the legacy `CODEX_AGENT_IDENTITY` env-var fallback and hidden
`--with-agent-identity` CLI alias.
- Updated CLI error, status, and stdin prompts to use access-token
language.
- Added coverage for access-token env loading, CLI login failure
behavior, and renamed login status text.
## Validation
- `cargo test -p codex-login`
- `cargo test -p codex-cli`
- `just fix -p codex-login`
- `just fix -p codex-cli`
## 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>
Shijie Rao