Add subagents RFD

[Ololoshechkin]

Add subagents RFD

[nikita-ashihmin]

Implementation feedback from AIR and the reference adapter branches:

• feat: expose native subagents and async tasks claude-agent-acp#1017
• feat: add native ACP subagent sessions codex-acp#419

The canonical capability and lifecycle shape works for both adapters. We only needed _meta.jetbrains.air.capabilities = ["nativeSubagentSessions"] as a temporary negotiation bridge because the released SDKs currently remove the draft subagents fields. We treat this metadata as a compatibility bridge, not as an alternative protocol. It may help to make SDK preservation of the draft fields an explicit rollout prerequisite.

Two edge cases still seem underspecified:

1. The RFD defines orphan recovery during session/load, but it does not define the immediate Client state when a live connection ends or session/prompt returns without the required terminal update. Clients can otherwise choose failed, keep running forever, or show disconnected. Could the RFD require a local disconnected state for an unknown outcome and clarify whether the Client emits no wire update?

2. A child can own pending session/request_permission or session/create_elicitation requests when the child or its parent is cancelled. Could the RFD require those requests to resolve or cancel before subagent_state_update, and define how a Client handles a response that races with the terminal update? This prevents a terminal child from retaining a live control or moving that control to the wrong history.

One smaller compatibility note: the current RFD requires task. AIR now treats the older description field only as a temporary input fallback. New adapters emit task.