Improve agent documentation and experience

[vinistock]

Motivation

Our agent instructions weren't the best and I had been meaning to add some agent experts to assist with some aspects of the Ruby LSP development.

This PR:

• Rewrites the AGENTS.md file to provide more information focused on architecture
• Adds 3 agents: a VS Code extension expert, a language server protocol expert and a plugin system architect

I also modified our bin/test to run tests using bundle exec ruby -Itest instead of bundle exec rake as it's a bit faster.

[vinistock]

• Improve agent documentation and experience #3966 👈 (View in Graphite)
• main

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