Add engagement metrics to article archives

[gaotongxue23]

What changed

• Added an optional history --metrics mode for best-effort public reading, like, share, and comment counters from an authorized desktop WeChat session.
• Stores available counters in each article meta.json and includes raw metric records in the history manifest.
• Clarified that the skill is an acquisition and factual-data handoff layer, not a content-analysis layer.

Why

Downloaded article archives lacked the interaction data needed by downstream analysis workflows.

Validation

• py -3 -m py_compile plugins/wechat-article-agent/skills/wechat-article-agent/scripts/wechat_agent.py
• history --help confirms --metrics is available.
• Validated the metrics request against an authorized desktop WeChat session.