feat(agents): relay encrypted Codex subagent payloads to external providers

Codex sends subagent turns as encrypted payloads that external providers
cannot read. Relay them through the native endpoint before routing, with a
short-lived bounded cache, and fail closed when a payload cannot be relayed.

Also adds managed agent-concurrency markers to the Codex config, marks
providers with multiAgentVersion, and raises the decoded body limit so a
compressed context larger than the encoded request limit still passes.