agent registration should validate A2A capability before adding to registry

[lucharo]

Problem

Society registers agents into the registry without validating that the target actually speaks the A2A protocol. This applies across all transports (stdio, Docker, SSH). The result is a registry that contains agents that look valid but fail immediately on any interaction.

Expected behavior

Before adding an agent to the registry, Society should:

1. Attempt an A2A handshake or health check
2. If the target doesn't respond, either reject registration or mark it as unverified
3. Surface clear feedback about agent readiness

Current behavior

Any discovered CLI, container, or SSH host gets registered as long as the transport layer (process, container, SSH tunnel) exists — regardless of whether an A2A-compatible agent is running.

Suggestion

• Add a lightweight A2A handshake as part of registration
• Introduce agent status in society list (e.g., verified, unverified, unreachable)
• Allow society ping to update the status
• Example output:

NAME       TRANSPORT  STATUS        ENDPOINT
codex      stdio      unverified    /opt/homebrew/bin/codex
my-agent   docker     verified      http://172.17.0.2:5001
my-host    ssh        unreachable   http://localhost:9090