When a user runs an unknown command that matches a known official
extension (e.g. `gh stack` or `gh aw`), show an install suggestion.
In interactive TTY sessions, prompt the user to install it on the spot.
The official extension registry is hard-coded in pkg/extensions/.
Current entries are github/gh-aw and github/gh-stack. Teams can add
their extensions via PR. Install suggestions use an explicit github.com
host prefix for GHES compatibility.
Refs github/cli#220
Co-Authored-By: Copilot <223556219+Copilot@users.noreply.github.com>
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