stdio transport: subprocess exits before responding

[lucharo]

Problem

When sending a message to an agent registered with the stdio transport (e.g. Codex CLI), the subprocess exits immediately before producing an A2A response:

$ society send <agent> "Hi!"
error: sending request: stdio: subprocess exited before responding

Expected behavior

Society should either:

1. Detect that the target CLI doesn't speak A2A over stdio and report a clear error at registration/onboard time
2. Or provide guidance in the error message about what the target process needs to support (A2A JSON-RPC over stdin/stdout)

Current behavior

The agent gets registered without any validation that it actually speaks A2A. The error only surfaces at send time and gives no indication of the root cause.

Suggestion

• During onboard, validate that stdio agents respond to an A2A handshake before registering them
• If validation fails, warn the user that the CLI may not support A2A and skip registration (or register with a warning)