[meta] Align and enhance AGENTS.md instructions

[PeterDaveHello]

References:

• https://agents.md
• https://github.com/agentsmd/agents.md
• https://github.blog/ai-and-ml/github-copilot/how-to-write-a-great-agents-md-lessons-from-over-2500-repositories/

GitHub Copilot summary:

This pull request updates the testing and linting instructions in the AGENTS.md documentation to provide clearer guidance on running tests and checks, including new npm scripts and improved descriptions. The main changes are grouped below:

Testing Instructions Improvements:

• Clarified the descriptions for npm test and make test to specify which shells are supported and updated instructions for running ksh tests, marking them as experimental and referencing issue ksh doesn't support local #574.
• Added documentation for running specific test suites using new npm run commands, covering fast, slow, sourcing, install script, and installation tests for both Node and io.js.
• Expanded single test and suite examples using urchin, including install script, Node installation, and io.js installation test suites.

Linting and Documentation Checks:

• Introduced a new section detailing npm scripts for linting and documentation checks, including EditorConfig compliance, README table of contents verification, Dockerfile linting, executable permission checks, and markdown link validation.