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/exec/src/lib.rs

@@ -475,7 +475,6 @@ pub async fn run_main(cli: Cli, arg0_paths: Arg0DispatchPaths) -> anyhow::Result
         forced_login_method: config.forced_login_method,
         forced_chatgpt_workspace_id: config.forced_chatgpt_workspace_id.clone(),
         chatgpt_base_url: Some(config.chatgpt_base_url.clone()),
-        agent_identity_authapi_base_url: None,
     })
     .await
     {