docs: mark A2A subagents as experimental in subagents.md (#17863)

Author: adamfweidman

docs/core/subagents.md

@@ -171,11 +171,13 @@ If you need to further tune your sub-agent, you can do so by selecting the model
 to optimize for with `/model` and then asking the model why it does not think
 that your sub-agent was called with a specific prompt and the given description.
 
-## Remote subagents (Agent2Agent)
+## Remote subagents (Agent2Agent) (experimental)
 
 Gemini CLI can also delegate tasks to remote subagents using the Agent-to-Agent
 (A2A) protocol.
 
+> **Note: Remote subagents are currently an experimental feature.**
+
 See the [Remote Subagents documentation](/docs/core/remote-agents) for detailed
 configuration and usage instructions.