Remove secret vault import (closes #139)

`secret vault import` took only a vault without a long-term key, and
`secret init` and `secret vault create` give every vault one, so it
always failed. A mnemonic enters through those two commands, so the
command goes, with its tests and its README entry. The help and the
note of `secret generate mnemonic` named `secret import`, which imports
a file as a secret; they now name `secret init` and
`secret vault create`.

`secret vault import` now prints the help of `secret vault` and exits
0, as cobra does for any subcommand of `secret vault` it does not know.

Judgement call: TestWorkflowMismatch in internal/vault, which replayed
the import step by step, is deleted.

Model: opus-5-5

README.md

@@ -268,10 +268,6 @@ Selects an unlocker as the current default for operations.
 Imports a secret from a file and stores it in the current vault under the given
 name.
 
-#### `secret vault import [vault-name]`
-
-Imports a mnemonic phrase into the specified vault (defaults to "default").
-
 ### Encryption Operations
 
 #### `secret encrypt <secret-name> [--input=file] [--output=file]`
@@ -377,9 +373,7 @@ line or in a CI job, they end up in shell history and CI logs. `secret` unsets
 each one as soon as it has read it, so that the programs it runs itself, such as
 `gpg`, do not inherit it, but that erases nothing: the environment the process
 started with, and its memory, still hold the value. The interactive prompt,
-which every command except `secret vault import` offers when the variable is not
-set, is the safer default; `secret vault import` has no prompt and needs both
-variables.
+which every command offers when the variable is not set, is the safer default.
 
 ## Security Features