feat(gateway): add WeChat Official Account platform adapter

[teknium1]

Summary

Adds WeChat support using the official WeChat Official Account API — no third-party bridges or competitor dependencies.

Replaces PR #2425 which depended on OpenClaw's WeixinClawBot service.

How it works

The adapter runs an aiohttp webhook server. WeChat pushes messages to it via HTTP POST with XML payloads. Replies go back via the Customer Service Message API (JSON).

• Receive: WeChat pushes XML → adapter parses → standard MessageEvent
• Send: Agent response → JSON POST to api.weixin.qq.com/cgi-bin/message/custom/send
• Auth: AppID + AppSecret → access_token (auto-refreshed, 2h TTL)
• Verification: SHA1 signature handshake per WeChat spec

Configuration

WECHAT_TOKEN=your-verification-token
WECHAT_APP_ID=wx1234567890abcdef
WECHAT_APP_SECRET=your-app-secret
# Optional
WECHAT_PORT=8680
WECHAT_HOME_CHANNEL=openid_abc123

Integration points

File	Change
gateway/platforms/wechat.py	New adapter (351 lines)
gateway/config.py	Platform.WECHAT enum + env overrides
gateway/run.py	Adapter factory
cron/scheduler.py	deliver=wechat support
agent/prompt_builder.py	Plain-text platform hint

Note

WeChat Official Accounts require verification through WeChat's admin dashboard. Users need a Service Account (not Subscription Account) for the Customer Service Message API. This is China-market focused — international users may find WeChat Work/WeCom more accessible.

Test plan

• Adapter imports and initializes correctly
• No regressions in gateway/cron tests

[teknium1]

Holding off on merge — the adapter code works but WeChat Official Account API requires Chinese business registration, ICP filing, and verification that 99% of users can't get. No competitor has shipped WeChat support for the same reason.

The code is saved as a reference implementation. When a user with actual WeChat access wants to test, or when we add Enterprise WeChat (WeCom) support (more accessible, where the Tencent+AI momentum actually is), we'll revisit.