Check vault names in every command that takes one (closes #68)

A vault name may use only lowercase ASCII letters, digits, `.`, `-` and
`_`, and must not be empty, `.` or `..`; the error now states that rule.
`vault create`, `vault import`, `vault select`, `vault remove` and both
vault names of `mv` check the name as typed before building any path
from it. Before, `vault import ..` wrote a long-term key and an unlocker
into the state directory itself, and `vault select ..` made that the
current vault.

Model: opus-5-5

internal/vault/errors.go

@@ -17,9 +17,10 @@ var (
 		"derived public key does not match vault: mnemonic may be incorrect",
 	)
 
-	// ErrInvalidVaultName indicates a vault name that does not match the
-	// allowed pattern [a-z0-9.\-_]+. Composed as
-	// "invalid vault name '<name>': must match pattern [a-z0-9.\-_]+".
+	// ErrInvalidVaultName indicates a vault name that breaks the naming
+	// rule: only lowercase ASCII letters, digits, '.', '-' and '_'; not
+	// empty, "." or "..". Composed by ValidateVaultName as
+	// "invalid vault name '<name>': <the rule>".
 	ErrInvalidVaultName = errors.New("invalid vault name")
 
 	// ErrVaultNotFound indicates the named vault does not exist. Composed