createAcpBackend (acp-backend.ts)

@fileoverview {@link AgentBackend} that drives an external coding agent over the Agent Client Protocol (ACP). OMA takes the ACP **client** role: it spawns the agent (a local CLI such as Gemini CLI or Claude Code) as a subprocess, exchanges JSON-RPC over stdio, and turns one `se

/**
 * @fileoverview {@link AgentBackend} that drives an external coding agent over the
 * Agent Client Protocol (ACP).
 *
 * OMA takes the ACP **client** role: it spawns the agent (a local CLI such as
 * Gemini CLI or Claude Code) as a subprocess, exchanges JSON-RPC over stdio, and
 * turns one `session/prompt` turn into a {@link RunResult}. This lets an ACP agent
 * sit in the same task DAG as LLM agents — with shared memory, cascade-on-failure,
 * and token budget — because it satisfies the same {@link AgentBackend} contract.
 *
 * Requires the optional peer dependency `@agentclientprotocol/sdk`, loaded lazily
 * so the three-runtime-dependency promise holds for consumers that never use ACP.
 * Reached only through `AgentConfig.backend` (see {@link Agent}) or the
 * `@open-multi-agent/c

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