feat: add A2A protocol channel for agent-to-agent communication

[zeroasterisk]

Summary

Adds @flue/a2a — a channel adapter that makes any Flue agent speak the A2A (Agent-to-Agent) protocol for inter-agent discovery and messaging.

Why

The A2A protocol enables interoperability between AI agents across different frameworks. With this channel, Flue agents can be discovered and messaged by any A2A-compatible client — including agents built with ADK, LangChain, CrewAI, or other frameworks.

What it does

• Agent Card: serves /.well-known/agent-card.json so other agents can discover this agent's capabilities
• message/send: accepts A2A JSON-RPC messages and routes them to the Flue agent
• Task lifecycle: tracks task state (submitted → working → completed/failed)
• Skills: maps Flue skills to A2A skill descriptors

How to test

cd packages/a2a && npm install && npm start

# Discover
curl http://localhost:3000/.well-known/agent-card.json

# Send message
curl -X POST http://localhost:3000/ \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"message/send","params":{"message":{"messageId":"1","role":"user","parts":[{"kind":"text","text":"Hello"}]}},"id":"1"}'

Files

• packages/a2a/src/index.ts — Channel factory and HTTP server
• packages/a2a/src/handlers.ts — A2A JSON-RPC method handlers
• packages/a2a/src/types.ts — A2A protocol type definitions
• packages/a2a/src/errors.ts — A2A error codes
• packages/a2a/README.md — Usage docs

A2A Protocol

The A2A specification is governed by the Linux Foundation with 150+ supporting organizations. This implementation targets protocol version 1.0.

[astrobot-houston]

Thanks for the contribution! We're closing this PR and moving the conversation to the existing thread: #285

We've moved to a model where bugs and feature proposals are discussed in issues/discussions before code review, so the community can help prioritize and shape the work. Your branch is linked from the new thread so the implementation isn't lost — please join us there to continue the conversation.

— astrobot 🤖