## Stack
This is PR 1 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
The simplified POC shape is one backend-owned task per Agent Identity
run. This PR makes the first layer match that final shape directly
instead of introducing task targets, caller-owned external task refs, or
intermediate wrappers that later PRs would need to undo.
What changed:
- keeps the `AgentAssertion` wire payload as `agent_runtime_id`,
`task_id`, `timestamp`, and `signature`
- exposes `register_agent_task` as the single task-registration helper
for both existing Agent Identity JWT auth and the ChatGPT-registration
path added later in the stack
- makes task registration send only the signed registration timestamp;
the backend owns the returned opaque task id
- removes the unused target/task-kind/external-task-ref surfaces from
`codex-agent-identity`
- keeps Agent Identity JWT JWKS lookup separate from agent/task
registration URL derivation
- updates Agent Identity JWT auth to register one run task during auth
construction and share that task across cloned auth handles
This PR intentionally does not enable ChatGPT-derived Agent Identity.
That opt-in and config gate are added in the next PR.
## Testing
- `just test -p codex-agent-identity`
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