feat: opt ChatGPT auth into agent identity (#19049)

## Stack

This is PR 2 of the simplified HAI single-run-task stack:

- [#19047](https://github.com/openai/codex/pull/19047) Agent Identity
assertion and task-registration primitives, including the shared
run-task helper used by existing Agent Identity JWT auth.
- [#19049](https://github.com/openai/codex/pull/19049)
Disabled-by-default ChatGPT auth opt-in that provisions/reuses persisted
Agent Identity runtime auth and its single run task.
- [#19051](https://github.com/openai/codex/pull/19051) Run-scoped
provider auth that uses one backend-owned task id for first-party
inference and compaction requests.

[#19054](https://github.com/openai/codex/pull/19054) collapsed out of
the active stack because the simplified design no longer needs a
separate background/control-plane task helper.

## Summary

This PR adds the disabled-by-default path for normal ChatGPT-login Codex
sessions to obtain Agent Identity runtime auth through the Codex
backend. Existing Agent Identity JWT startup mode remains a separate
path and does not require the feature flag.

What changed:

- adds the experimental `use_agent_identity` feature flag and config
schema entry
- adds an explicit `AgentIdentityAuthPolicy` so call sites choose
`JwtOnly` or `ChatGptAuth` instead of passing a bare boolean
- stores standalone Agent Identity JWT credentials separately from
backend-registered Agent Identity records
- persists the registered Agent Identity record, private key, and single
run task id in `auth.json` so process restarts reuse the same identity
- derives the agent/task registration base URL from ChatGPT/Codex auth
config while keeping JWT JWKS lookup separate
- provisions and caches ChatGPT-derived Agent Identity runtime auth when
`use_agent_identity` is enabled
- reuses the shared run-task registration helper from PR1 rather than
adding a second task-registration path

This PR intentionally does not switch model inference over to
`AgentAssertion` auth. The provider-auth integration lands in the next
PR.

## Testing

- `just test -p codex-login`

codex-rs/login/tests/suite/auth_refresh.rs

@@ -1217,6 +1217,7 @@ impl RefreshTokenTestContext {
             codex_home.path().to_path_buf(),
             /*enable_codex_api_key_env*/ false,
             AuthCredentialsStoreMode::File,
+            /*forced_chatgpt_workspace_id*/ None,
             /*chatgpt_base_url*/ None,
             AuthKeyringBackendKind::default(),
         )

codex-rs/login/tests/suite/logout.rs

@@ -201,6 +201,7 @@ async fn auth_manager_logout_with_revoke_uses_cached_auth() -> Result<()> {
         codex_home.path().to_path_buf(),
         /*enable_codex_api_key_env*/ false,
         AuthCredentialsStoreMode::File,
+        /*forced_chatgpt_workspace_id*/ None,
         /*chatgpt_base_url*/ None,
         AuthKeyringBackendKind::default(),
     )