An empty currentvault, or one naming a vault that does not exist, made
`secret list` report an empty vault with exit 0 and `secret add` fail on a
missing directory. ResolveVaultSymlink now refuses an empty or invalid
name, or one with no directory under vaults.d, with an error that wraps
ErrVaultNotFound, names the file and says to run
`secret vault select <name>`. Every command that uses the current vault
gets it; `secret vault list` and `secret info` still run.
newSizeTestVault wrote a full path into currentvault, which resolved only
because the last path element was taken as the name; it now relies on
CreateVault selecting the vault.
Model: opus-5-5
sneak