chore: automate snippet validation via npm test

[EndlessTrax]

Summary

• add scripts/validate-snippets.js to validate snippet JSON, required fields, catalog prefix namespaces, and duplicate prefixes
• add npm test in package.json to run snippet validation from a single command
• update PR CI workflow to use npm test instead of inline node -e parsing
• update contributor docs in README.md and .github/copilot-instructions.md to match the new validation flow

Validation

• npm ci
• npm test
• npx vsce package

Closes #48

[Copilot]

Pull request overview

This PR formalizes snippet-catalog validation as a first-class workflow step by introducing a dedicated Node.js validation script and wiring it into npm test, CI, and contributor documentation for this VS Code snippets extension.

Changes:

• Add scripts/validate-snippets.js to validate snippet JSON structure, required fields, prefix namespaces, and duplicate prefixes across catalogs.
• Add npm test in package.json to run the validation script.
• Update GitHub Actions CI and docs (README + Copilot instructions) to use the new npm test validation flow.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
scripts/validate-snippets.js	New validation script for snippet catalogs and prefix rules.
README.md	Updates CI/contribution docs to reference npm test.
package.json	Adds scripts.test to run the validator.
.github/workflows/ci.yml	Switches snippet validation step to npm test.
.github/copilot-instructions.md	Updates documented local/CI validation commands to npm test.