Update: add local skill validation workflow

[dwk-123]

Summary

• Make tools/quick_validate.py stdlib-only and add repo-wide --all validation
• Fix the two existing structural validation issues without changing website-deploy's v15-v16 compatibility
• Add local GitHub Actions validation plus source verification docs for official Frappe/Frappe Docker URLs
• Document the exact splitlines() <= 500 line-count rule in CONTRIBUTING.md

Validation

• python3 tools/quick_validate.py skills/source/core/frappe-core-database
• python3 tools/quick_validate.py skills/source --all
• verified 61 skills discovered
• verified no SKILL.md exceeds 500 splitlines
• packaged smoke tests for frappe-ops-deployment and frappe-core-database
• git diff --check

Notes

• The validator intentionally checks structure only; source accuracy evidence is recorded in docs/validation/frappe-source-verification-2026-06-24.md
• No runtime DB/schema/API changes