feat: support ephemeral ACP sessions

[hokupod]

Summary

• support _meta.codex.ephemeral: true on ACP session/new
• forward the opt-in as thread/start.ephemeral: true
• reject non-boolean values with ACP invalidParams
• document the extension and cover persisted-session compatibility

Motivation

Automation clients can create one-off ACP sessions whose Codex threads do not need to appear in stored thread listings or remain available for resume. The generated ThreadStartParams type already exposes the App Server ephemeral field, but codex-acp did not provide an ACP request path to it.

Kyoso is a multi-agent code review tool that runs Codex reviewers through ACP. Each review creates a one-off session that is never resumed, but currently remains as a persisted task in Codex App. After this change is released, Kyoso will opt these reviewer sessions into ephemeral storage.

Compatibility

• missing _meta.codex.ephemeral keeps the existing persisted-session behavior
• ephemeral: false also keeps the existing persisted-session behavior
• unrelated keys under _meta.codex remain ignored
• generated App Server types are unchanged

Base: agentclientprotocol/codex-acp@2dccf45b53a33089d5b6e82508a1887aec8b20cc

Validation

• npm run typecheck
• npm test -- src/__tests__/CodexACPAgent/CodexAcpClient.test.ts — 102 passed
• npm test — 405 passed, 28 skipped
• npm run build
• git diff --check