Introduce canary, RC, and stable release channels

The Tuist CLI is now distributed through three release channels: canary (per-commit prereleases), release candidate (soaked prereleases), and stable (promoted after a clean soak). Stable remains the recommended default, while canary and RC builds are opt-in prereleases. This gives teams a slower-moving recommended install and a clear path for early adopters to test per-commit builds.