Add named storage profiles on top of `TART_HOME`

[TyceHerrman]

Problem

TART_HOME already allows Tart's VMs and cache to live on an external drive, as documented through #221 and #845. It works well when a user has one alternate storage root.

Using multiple roots is less ergonomic: callers must export or prefix TART_HOME with a path for every command, remember which root contains each VM, and reproduce those paths across scripts. The path-based approach proposed in #199 was closed in favor of TART_HOME, but named roots could remain fully compatible with that decision.

Proposal

Allow users to register storage roots by name and select one per command. For example:

tart storage add internal ~/.tart
tart storage add external /Volumes/VMs/tart
tart storage list

tart --storage external list
tart --storage external run review-macos

This would be a small configuration layer over the existing Tart home abstraction rather than a new VM format.

Suggested behavior

• Preserve the current default storage location and TART_HOME behavior.
• Define clear precedence between --storage, TART_HOME, and the default location.
• Apply the selected storage root consistently to VMs, OCI cache, temporary files, pruning, and free-space checks.
• Namespace duplicate VM names by storage profile.
• Validate that configured locations exist and are writable.
• Warn when a non-APFS destination prevents efficient copy-on-write cloning.
• Removing a profile should remove only its configuration, never its VMs or cache.

Alternatives considered

Shell aliases such as TART_HOME=/Volumes/VMs/tart tart ... work, but they duplicate absolute paths and provide no discovery mechanism for tools or users. Symlinking ~/.tart also makes switching roots awkward and has caused support issues in the past.

Would named storage profiles be acceptable as an additive interface over TART_HOME?