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

README.md

@@ -91,6 +91,9 @@ Lists all available vaults. The current vault is marked.
 
 Creates a new vault with the specified name.
 
+**Vault Name Format:** only lowercase ASCII letters, digits, `.`, `-` and `_`
+are allowed, and a name must not be empty, `.` or `..`.
+
 #### `secret vault select <name>`
 
 Switches to the specified vault for subsequent operations.