Three release channels: canary, rc, and stable

CLI distribution is now structured into three release channels: canary (per-commit prereleases on main, opt-in), release candidate (cut from main onto releases/X.Y.x, soaked before promotion, opt-in), and stable (promoted from a clean RC, recommended install). Stable releases are reached deliberately instead of cutting a stable tag on every commit. The recommended install moves slowly so upgrades no longer absorb unrelated changes.