Nightly channel and upgrade-source invariants

Nightly is a persistent channel: sentry cli upgrade nightly switches and updates immediately, and later bare upgrades remain nightly until stable is selected. Nightly always uses standalone curl/GitHub/GHCR semantics regardless of detected installation method, including OCI authentication, manifests, version annotations, platform gzip layers, and pinned-version validation. Preserve the selected source through validation, delta lookup, download, and installation. Migrate package-manager and Homebrew installations to standalone with the proper uninstall hint. Stable alone uses installation-method routing. Package-manager upgrades require network access; reject explicit non-curl offline upgrades with the established UpgradeError reason and message, and limit offline fallback to curl-installed binaries. Retain the newest 30 nightly-* tags and remove older nightly tags with corresponding patch-* tags. Test equivalent nightly behavior across installation methods.