docs: add FOOTER.md and Documentation & Skills section to all READMEs

[pyramation]

Summary

Introduces FOOTER.md as a single source of truth for the shared footer across all package READMEs. Adds a new "Documentation & Skills" subsection with npx skills add install commands so AI coding agents can discover and install Constructive skills.

Changes:

• New FOOTER.md at repo root containing: Related Tooling (updated links), Documentation & Skills (npx skills add commands), and Disclaimer
• All 26 package READMEs + root README updated to use the standardized footer content from FOOTER.md
• ltree-helpers/README.md — added footer (previously missing entirely)
• Related Tooling links updated to correct paths (pgpm/pgpm, postgres/pgsql-test, etc.)

New section added to each README:

### 📚 Documentation & Skills
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

Companion PR (merged): constructive-io/constructive#1272

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

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