Commands fail when currentvault names no vault (closes #156)

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` still lists the vaults.

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

README.md

@@ -114,7 +114,10 @@ are allowed, and a name must not be empty, `.` or `..`.
 
 #### `secret vault select <name>`
 
-Switches to the specified vault for subsequent operations.
+Switches to the specified vault for subsequent operations. When `currentvault`
+is empty or names a vault that does not exist, every command that uses the
+current vault fails with an error that names the file and says to run
+`secret vault select <name>`; `secret vault list` still lists the vaults.
 
 #### `secret vault remove <name> [--force]` / `secret vault rm` ⚠️ 🛑