[FEATURE]: Native A2A support (serve sessions over A2A, delegate to remote A2A agents) #62903

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Describe the enhancement you want to request

Summary

Add native support for A2A v1.0, the open agent-to-agent protocol hosted by the Linux Foundation, in both directions: serve opencode as an A2A agent, and delegate to remote A2A agents. A2A complements what opencode already supports: MCP connects it to tools, ACP to editors, and A2A to other agents.

Benefits

Inbound

• Orchestrators built on ADK, LangGraph, CrewAI or any A2A client can use opencode as a coding sub-agent without custom glue.
• Long tasks map to A2A tasks: callers stream progress, poll with GetTask and stop with CancelTask.
• Permission prompts reach the caller as input-required instead of being auto-approved or blocking.
• opencode keeps its own workspace, tools, keys and permissions; the caller sees only messages and artifacts.

Outbound

• Delegate to specialized remote agents (a review service, an internal docs agent) by adding a URL, instead of a custom tool or MCP wrapper per agent.
• The Agent Card gives the model the agent's description, skills and auth requirements.
• Follow-ups continue the same remote conversation through contextId/taskId.

Proposed design

Inbound: opencode a2a

• New command in packages/cli/src/a2a/, built like opencode acp, serving the Agent Card and SendMessage, SendStreamingMessage, GetTask, CancelTask through @a2a-js/sdk.
• An AgentExecutor maps each contextId to one session and streams session events as task updates.
• permission.asked and form.created become TASK_STATE_INPUT_REQUIRED; the reply on the same taskId goes to permission.reply or session.form.reply.
• CancelTask calls session.interrupt.

Outbound

• A plugin with a remote_agent tool; remote agents are listed by Agent Card URL in the plugin options. Later, subagent definitions could accept a card URL directly.

We can implement and maintain this as two PRs, inbound then outbound.