Separate canary / rc / stable release channels for the CLI

CLI distribution is split across three channels: canary on every CLI-touching commit to main, opt-in release candidates on a releases/X.Y.x line after deliberate cut, and stable once an RC has soaked. tuist@latest / tuist@4.200 now only receive backported fixes, so upgrading does not force a batch of unrelated changes. The server recommendation also stops advertising a canary as the recommended install, and release tooling refuses to promote a stable before a fresh RC has soaked the current HEAD. A new docs page documents the channels, pinning, and backport policy.