Add Seven Powers strategy skill

[louzhedong]

Summary

• add a standalone seven-powers skill to pm-product-strategy for durable competitive advantage analysis
• cover the benefit/barrier test, all seven powers, false positives, output process, and report template
• update plugin and marketplace metadata so the strategy plugin count and description stay current

Why

This addresses #35 by adding Hamilton Helmer's 7 Powers as a focused strategy skill that complements Porters Five Forces and the Product Strategy Canvas defensibility section without adding a new command yet.

Validation

• python3 validate_plugins.py
• git diff --check