Add asdf support to Platform Provisioning Manifests

[darkn3rd]

Currently each language has its own single-purpose version manager in the manifest schema - pyenv (Python), rbenv (Ruby), sdkman (JVM languages) - with no way to express asdf, the general-purpose, plugin-based version manager that can replace any of them depending on which plugins a manifest installs (ruby, python, java, golang, etc.).

Depends on #16 - asdf needs to be opt-in alongside pyenv/rbenv/sdkman/direct system installs (you don't want multiple version managers for the same language installing at once), which needs real --select/--exclude filtering, not just documentation-only tags:. asdf's own package entries will carry tags: [asdf].

• Add asdf as a new package type: in resolve_order.rb's PACKAGE_TYPES
• Add install-command handling - asdf's install is git-clone + shell-hook + per-language asdf plugin add/asdf install, not a one-line package-manager call, so this needs a script:-based bootstrap (similar to ubuntu22_rbenv) plus separate asdf install <language> <version> steps per language a manifest wants asdf-managed
• 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 asdf wouldn't apply there (asdf itself doesn't support native Windows either, only WSL/POSIX shells)
• Decide tagging granularity - a single tags: [asdf] for the bootstrap step, but should each asdf-managed language also carry a language-specific tag (e.g. [asdf, ruby-version-manager]) so --select/--exclude can swap asdf in for just one language rather than all of them at once?
• Document asdf 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 asdf-installed languages transparently (it detects binaries by name, not by install method)

This is additive/optional - existing platforms already install each language fine without asdf. This is about giving manifest authors the choice of a single unified version manager where they want it, once #16 makes that choice actually selectable.