Advertise support for message-specific session forks #640

Hello,
codex-acp supports forking at an earlier message through _meta.jetbrains.air.fork, added in #435. However, I couldn’t find a capability marker that distinguishes this from ordinary whole-session forking.

This came up while discussing this feature in Emacs Agent Shell

Could codex-acp advertise this extension during initialization? ACP recommends advertising extension capabilities through _meta.

That would let clients enable message-specific forking without relying on adapter names or version checks, and avoid silently copying the entire conversation when an adapter ignores the fork-point metadata.