Tuist CLI now publishes through canary, rc, and stable release channels

Tuist now ships through three channels: canary (every CLI-touching commit, opt-in), release candidate (cut from releases/X.Y.x and soaked, opt-in), and stable (promoted after the soak, default for tuist@latest and tuist@X.Y). tuist install and Homebrew respect the prerelease flag so default resolution no longer picks up canary builds. A new Release channels docs page covers pinning strategy and the backport policy.