Add backend development skill

[egagofur]

Summary

• Add a framework-agnostic backend-development skill for Claude, Grok, Gemini, and Codex-compatible .agents hosts
• Include API design, query/database, and security reference docs for progressive disclosure
• Wire the skill into sync-hosts, doctor checks, package assets, and host compatibility tests

Verification

• node --test tests/sync-hosts.test.js tests/skill-host-compat.test.js
• npm run doctor
• npm run audit:package
• npm test

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