Add rvm support to Platform Provisioning Manifests

[darkn3rd]

Currently the only Ruby version manager in the manifest schema is rbenv (used in ubuntu2204.yml) - other platforms just install a single system/package Ruby directly (macos.yml's brew: ruby, windows.yml's choco: ruby, cygwin.yml/msys2.yml's native package). RVM is a well-known alternative Ruby version manager, primarily for POSIX/bash environments, and isn't representable in the manifest schema at all right now.

Depends on #16 - RVM steps need to be opt-in alongside rbenv/the system Ruby (you don't want all three installing by default), which means they need real --select/--exclude filtering, not just documentation-only tags:. RVM's own package entries will carry tags: [rvm], matching the existing [rbenv]/[pyenv]/[sdkman] convention.

• Add rvm as a new package type: in resolve_order.rb's PACKAGE_TYPES
• Add install-command handling - RVM's own install is a multi-step curl+gpg bootstrap, not a one-line package-manager call, so this likely needs a script:-based approach (similar to ubuntu22_rbenv) rather than a simple bash_install/powershell_install case
• Decide which platform(s) actually get it - bash-capable platforms only (ubuntu2204.yml, macos.yml, cygwin.yml, msys2.yml); native Windows has no bash, so RVM wouldn't apply there
• Document rvm in scriptbox/config/README.md's "Version Managers" list, alongside pyenv/rbenv/sdkman
• Confirm whether scriptbox/scripts/verify_commands.rb needs any change, or whether it already covers RVM-installed Ruby transparently (it detects ruby/rake by binary name, not by install method)

This is additive/optional - existing platforms already install Ruby fine without RVM. This is about giving manifest authors the choice of RVM as a package type where they want it, once #16 makes that choice actually selectable.