chore: add reusable local checks

[vku2018]

Closes #1

Summary:

• add reusable local validation scripts: check, check:syntax, check:json, check:generated, and check:safety
• move CI's inline JSON and safety checks into scripts/check-json.mjs and scripts/check-safety.mjs
• update CI to call the npm scripts and document npm run check in the README

Validation:

• npm run check
• git diff --check

Note: npm run check regenerated rules/rules.json and shared/config.js locally as expected, but they had no content diff and are not included in this PR.