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

@@ -277,10 +277,6 @@ 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]`
@@ -386,9 +382,7 @@ 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.
 
 The prompt asks on the terminal the command runs in, not on stdin, so it also
 asks when stdin is a pipe, as in `echo value | secret add name --force` or