feat: add generic A2A client service

[rodaddy]

Goal

Add a generic a2a-client service to mcp2cli for standard Agent-to-Agent JSON-RPC/SSE interactions.

Context

The Herdr/MonkeyProof architecture should talk to remote workers through A2A where possible. MonkeyProof has a facade, Bilby has an A2A server, and King agents expose A2A surfaces. A generic client avoids one-off curl commands and makes agent-to-agent work repeatable.

Proposed Tools

• get_agent_card
• send_message
• send_streaming_message
• send_followup
• cancel_task
• optional list_known_agents if backed by config/discovery

Requirements

• Include A2A-Version: 1.0 automatically unless overridden.
• Support endpoint URL, message text, task id, metadata, and timeout.
• Preserve structured task/artifact/status output.
• Support streaming output when practical; if not, provide a clear polling/read fallback.
• Keep endpoint credentials in config/secret stores, not prompts or committed files.
• Include examples for:
  • MonkeyProof A2A on CT 120;
  • Bilby A2A;
  • King agents A2A where applicable.

Acceptance Criteria

• mcp2cli a2a-client --help lists the tools.
• mcp2cli schema a2a-client.send_message documents parameters.
• Can retrieve an agent card from a configured endpoint.
• Can submit a task and return task status/artifacts.
• Can send a follow-up to an existing task id.
• Can cancel a task where the endpoint supports it.

[rodaddy]

Part of the Herdr-controlled remote worker orchestration epic:

• rodaddy/rtech-infra#133

This work should feed the PRD/implementation plan there and be coordinated from a Herdr controller session.