[Feature]: Proactive 消息支持自定义发送频道（如飞书、钉钉等）

## Summary

Proactive 主动消息支持自定义发送频道（飞书、钉钉等），而非仅限 Console 控制台。

## Component(s) Affected

- [ ] Core / Backend (app, agents, config, providers, utils, local_models)
- [ ] Console (frontend web UI)
- [x] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Problem / Motivation
用户日常通过飞书/钉钉与 QwenPaw 交互，不会一直盯着 Web 控制台。Proactive 推送的消息希望能直接发到 IM 工具，而不是需要回头去控制台找。

## Proposed Solution
1. 配置文件支持：`agent.json` 添加 `proactive.channel` 字段
2. 魔法命令支持：`/proactive --channel feishu`

## Alternatives Considered

## Additional Context
文档：https://qwenpaw.agentscope.io/docs/memory-evolving-and-proactive

## Willing to Contribute
- [ ] I am willing to open a PR for this feature (after discussion).