Claude Code and GitHub Copilot are now opt-in per user

The Claude Code and GitHub Copilot execution harnesses require a personal subscription login — offering them to every user by default only produced confusing "Not logged in" dead ends for people who had never chosen them.

They are now install-gated per user:

• By default, the chat's harness picker offers the native MeshWeaver harness only. Nobody lands on a CLI harness without asking for it.
• Opting in is a Store install: installing the Claude Code (or GitHub Copilot) plugin places the harness in your picker only, ready for /login.
• Uninstalling removes it again — including for threads that had already selected it, which fall back gracefully to the standard agent path instead of erroring.

Existing threads or composers still pointing at a CLI harness they no longer have simply run on the default harness; nothing breaks.