feat(provider): implement ACP Provider for OpenCode integration

[aaronwong1989]

• 背景: OpenCode 已原生支持 ACP（opencode acp 命令）
• 目标: 实现 ACP Provider，让 HotPlex 通过 ACP 协议连接 OpenCode
• 架构:

  ChatApp (Slack/TG)
        ↓
  HotPlex Engine
        ↓
  ACP Provider (implements Provider interface)
        ↓
  HTTP Client → OpenCode ACP Server (stdio)
  

• 关键实现点:
  • 新增 provider/acp_provider.go
  • 实现 BuildInputMessage：构造 ACP Prompt JSON
  • 实现 ParseEvent：解析 nd-JSON 事件流
  • Session 映射：默认隔离 + 显式绑定
  • 配置优先级：Flags > Env > YAML
• 参考:
  • OpenCode ACP 文档: https://opencode.ai/docs/zh-cn/acp/
  • OpenClaw 实践: docs/research/openclaw-acp-deep-dive.md
• 关联: Resolves feat(provider): track Claude Code ACP support (upstream #6686) #355 (跟踪上游 ACP 进展)