feat: Add Cursor as built-in agent (standard ACP JSON-RPC 2.0)

[altryne]

Summary

Cursor launched ACP support today (March 4, 2026): https://cursor.com/blog/jetbrains-acp

The Cursor CLI exposes a standard ACP server via agent acp (~/.local/bin/agent acp), speaking JSON-RPC 2.0 over stdio.

Problem

acpx uses a custom JSON lines protocol for its 5 built-in agents. When configuring Cursor as a custom agent in ~/.acpx/config.json, acpx spawns the process but fails with ACP_TURN_FAILED because it speaks its custom protocol, not standard ACP JSON-RPC 2.0.

Cursor ACP Details

• Binary: agent acp (installed by Cursor.app to ~/.local/bin/agent)
• Protocol: Standard ACP JSON-RPC 2.0 over stdio
• Auth: cursor_login method
• Docs: https://cursor.com/docs/cli/acp

Request

Add cursor as a built-in acpx agent, either by:

1. Creating a cursor-acp adapter (like codex-acp)
2. Adding native standard ACP support to acpx (Feature: AcpRuntime plugin for standard ACP agents (Kiro, Copilot, Cline, and 19+ others via JSON-RPC 2.0) openclaw#28511)

Option 2 would also unlock Kiro, Copilot, Cline, and 19+ other ACP Registry agents.

Context

• Related: Feature: AcpRuntime plugin for standard ACP agents (Kiro, Copilot, Cline, and 19+ others via JSON-RPC 2.0) openclaw#28511
• ACP Registry: https://agentclientprotocol.com/get-started/registry
• Tested: macOS arm64, acpx 0.1.15, Cursor 2.6.11