refactor: introduce composable harness architecture

[oyjt]

背景

参考 deepseek-ai/deepseek-harness 的插件化、capability seam、可逆注册、事件驱动与执行前策略机制，对 Agent Workbench 开始架构升级。

本 PR 不复制 DeepSeek Harness 的完整 Cordis/agent loop，而是保留 Workbench 现有的 Runtime Adapter 控制平面定位，先建立轻量 harness 内核，作为后续拆分 src/App.tsx 与 server/index.mjs 的稳定边界。

本次改动

• 新增前端/共享 harness contracts：
  • AgentHarness
  • CapabilityRegistry
  • HarnessEventBus
  • reversible plugin registration
  • risk policy seam
• 新增 server harness：
  • capability registry
  • plugin lifecycle
  • allow / ask / deny policy
  • guarded capability execution pipeline
  • capability lifecycle events
• 增加 Node 原生测试，覆盖：
  • capability 可逆注册
  • plugin 卸载清理
  • medium-risk 审批后执行
• 增加 pnpm test / pnpm verify
• 新增 docs/architecture/harness-refactor.md，明确迁移目标、边界和分阶段方案

后续迁移顺序

1. 拆 server/index.mjs：db/schema → repositories → events → approval → connectors/runtime → HTTP/SSE
2. 将 CLI/MCP 接入统一 capability/policy/approval pipeline
3. 拆 src/App.tsx：workflows → tasks/runs → approvals → agents → connectors/skills → knowledge/artifacts
4. 增加 capability scope / profile / loadout
5. 让 durable run events 成为运行控制台、审计、恢复和回放的事实源

设计取舍

• 保留 Runtime Adapter，不在 Workbench 内重写成熟 coding agent 的模型循环。
• 不引入完整 Cordis，控制当前项目复杂度。
• 先创建 seam 与测试，再逐模块迁移，避免大爆炸式重写和功能回退。

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