Document how to configure these plugins to share within a repo

[heaths]

1. Claude Code: https://code.claude.com/docs/en/plugins-reference#plugin-installation-scopes
2. Copilot CLI: can't find anything equivalent, but might be able to use use remote references like I do in this repo for the azsdk-samples MCP, along with https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/plugins-creating#creating-a-plugin

In both cases - expectedly and desirably - the plugins are installed by default. Like VSCode extensions - specifically .vscode/extensions.json - they are merely recommended or, more likely with CC or Copilot, just available. Maybe they get "advertised" when the CLI starts. If not, that'd be a nifty idea.