docs: add npx skills add commands to FOOTER.md and pgpm CLI README

[pyramation]

Summary

Adds npx skills add install commands to two places:

1. FOOTER.md — new install block under the existing "Documentation & Skills" section:

   npx skills add constructive-io/constructive-skills
   npx skills add https://github.com/constructive-io/constructive --skill pgpm
   npx skills add https://github.com/constructive-io/constructive --skill constructive-testing

   This propagates to all package READMEs that use the shared footer.

2. pgpm/cli/README.md — new "AI Agent Skills" section at the bottom with the pgpm-specific install command and a pointer to the full platform skills.

Link to Devin session: https://app.devin.ai/sessions/03c02f34a67c4e869e8abbaacc71c5c5
Requested by: @pyramation

[devin-ai-integration]

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