Hide Chat panel when Assistant is disabled

[jmcphers]

This change hides the Chat panel entirely when Positron Assistant is disabled. The approach taken is:

• Never show the Chat panel unless there are chat agents registered. The upstream configuration shows the panel in other contexts, such as when an incorrect version of Copilot is installed -- not something we need to worry about since we bundle Assistant.
• Do not register any chat agents when Assistant is disabled. This is necessary since the default agent comes from a built-in extension and is therefore always registered.

Addresses #6828.

QA Notes

A reload or restart is required to apply the setting.

[timtmok]

LGTM

If the preference ever becomes user facing, it would be good to have a ChangeObserver on it to notify that a restart is required.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

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