Add Google A2A protocol support alongside XMTP messaging

[0xtus]

Description

Add support for Google's Agent-to-Agent (A2A) protocol as an alternative messaging transport alongside XMTP.

Background

Google's A2A protocol is an emerging standard for agent-to-agent communication. Supporting both A2A and XMTP gives agents maximum interoperability.

Scope

• Add A2A agent card generation from trust registry metadata
• Implement A2A task/message exchange
• Allow agents to be discoverable via both XMTP and A2A
• New tools: azeth_a2a_send, azeth_a2a_receive, azeth_a2a_agent_card

Prior art

The existing XMTP messaging tools in src/tools/messaging.ts provide the pattern to follow.