feat: support ephemeral ACP sessions

[hokupod]

Summary

Allow automation clients to create one-off ACP sessions backed by in-memory Codex threads. On session/new, _meta.codex.ephemeral: true is forwarded to thread/start as ephemeral: true.

Kyoso runs Codex reviewers through ACP and does not resume those review sessions. This opt-in lets such clients avoid retaining one-off reviews as persisted Codex tasks. The adapter needs to forward the storage choice when it creates the Codex thread; filtering the client's session list alone would not change thread persistence.

Compatibility

• Missing metadata or ephemeral: false preserves the existing persisted-session behavior.
• Non-boolean ephemeral values are rejected with ACP invalidParams before thread creation.
• Unrelated keys under _meta.codex remain ignored.
• The opt-in is documented in README. Ephemeral sessions are not intended to be listed or resumed.
• The feature uses the existing generated ThreadStartParams.ephemeral field and requires no schema changes.

Validation

Validated with upstream main at effb0fe670a49dfbb5071764b5f8a2e3c09e2393 incorporated into head c9e698e86b0f88566ba31fcfe3688f2b5c5d5366:

• npm run typecheck — passed.
• npm test — 614 passed, 26 skipped, including opt-in, default/false compatibility, and invalid-value coverage.
• npm run build — passed.
• npm run bundle:all with CI's Bun 1.3.11 — all six targets passed.
• git diff --check effb0fe670a49dfbb5071764b5f8a2e3c09e2393 HEAD — passed.
• Real Codex smoke test using a temporary adaptation of the repository's run-codex runner with ephemeral metadata — thread/started reported thread.path = null, and the prompt completed with end_turn. This was a focused smoke test; the full live-provider E2E suite was not run.

GitHub CI and Conventional PRs currently report action_required with zero jobs; hosted checks have not run on this head.

[hokupod]

@nikita-ashihmin I've resolved the conflicts with main and updated the validation results at head c9e698e. Local checks pass: 614 tests passed, 26 skipped, plus typecheck, build, and all six binary targets. A real Codex smoke test confirmed thread.path = null and normal prompt completion.

The CI and Conventional PRs runs show action_required with zero jobs. Could you approve the fork workflows when you have a chance?

Could you also confirm whether _meta.codex.ephemeral on session/new is an acceptable API shape? This lets one-off ACP review sessions use Codex's existing in-memory thread option while preserving default behavior. I'm happy to adjust the metadata namespace or capability advertisement to the project's preferred convention.