docs: clarify that GitHub Copilot Chat (VS Code/JetBrains) is supported (#497) (#526)

"GitHub Copilot (editor)" was buried in a comma-separated adapter
list with no indication of which product it meant, so readers asking
"does this support VS Code Copilot?" had no way to tell from the
README. Spell out VS Code/JetBrains/Visual Studio explicitly and
disambiguate from the separately documented Copilot CLI.

Closes #497