rzv CLI 0.9.0: Go rewrite and single-file downloads

rzv 0.9.0 is rewritten in Go and ships as single-file downloads for Linux, Mac and Windows.

• rzv is rewritten in Go. Each download is one file with no runtime dependencies, and Linux no longer needs libatomic1.
• rzv accounts is now rzv workspaces. The old name still works, and --workspace replaces --account, which also still works.
• Downloads are available for Linux arm64, Intel Macs and Windows x86-64, alongside Linux x86-64 and Apple Silicon Macs.
• rzv whoamiShows the signed-in identity, environment, session expiry and selected workspace.
• rzv login --env prd|sharedChooses the environment. Production is the default, and --no-browser is available.
• RZV_API_KEY works without a stored sign-in when you pass --workspace.
• rzv repositories listLists the repositories available to your workspace.
• Release notes are on GitHub at Runzivo/rzv-cli.