gh stack subcommands not tab-completable in zsh

`gh stack` shows up as a completion option after `gh `, but tabbing after `gh stack` returns nothing.

Start by reproducing completion after `gh stack` in zsh, then trace the gh-stack completion entry point and how its subcommands are exposed. Done means each `gh stack` subcommand appears when tab-completing in zsh, with coverage for the observed behavior if the repository provides completion tests.

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