upgrade --offline curl-only contract

sentry cli upgrade --offline is supported only for curl-installed binaries; reject npm, pnpm, Bun, Yarn, and Homebrew because package-manager installation always requires network access. In curl offline mode, binpatch checks its cache without resolving a source or touching the network and reports an offline miss when absent. Pinned nightly validation always uses curl/GitHub lookup regardless of the detected install method because nightly builds are GitHub-only.