Update Node.js engine, workflows, and test coverage

[coliff]

• Lowered the minimum Node.js engine requirement to 18 in package.json and added Node.js 18 to the test matrix.
• Updated CodeQL GitHub Actions to v3.30.2 and improved documentation for GitHub Actions usage.
• Removed a test that checked for rule presence in rules/index.mdx. (because we don't put rules live on the website until new version is published).

[Copilot]

Pull Request Overview

This PR updates Node.js compatibility and GitHub Actions configurations while improving test coverage practices. The changes lower the minimum Node.js version requirement from 20 to 18, update CodeQL analysis actions to a newer version, and remove a test that was checking for rule presence on the website before publication.

Key changes:

• Lower Node.js engine requirement to version 18 and add Node 18 to CI test matrix
• Update CodeQL GitHub Actions from v3.30.1 to v3.30.2
• Remove test checking for rule presence in rules/index.mdx to align with publication workflow

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
package.json	Updated Node.js engine requirement from >=20 to >=18
.github/workflows/test.yml	Added Node.js 18 to the test matrix
.github/workflows/codeql-analysis.yml	Updated CodeQL actions from v3.30.1 to v3.30.2
.github/copilot-instructions.md	Added GitHub Actions documentation and improved formatting
test/rules/documentation.spec.js	Removed test checking for rule presence in rules/index.mdx

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View logs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (95e6a13) to head (abc6090).
⚠️ Report is 134 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #1719      +/-   ##
===========================================
+ Coverage   96.99%   100.00%   +3.00%     
===========================================
  Files           2         1       -1     
  Lines        1628         1    -1627     
  Branches      335         0     -335     
===========================================
- Hits         1579         1    -1578     
+ Misses         49         0      -49     

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