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

internal/vault/management.go

@@ -215,11 +215,11 @@ func processMnemonicForVault(
 // CreateVault creates a new vault and selects it as the current vault. When
 // mnemonic is not nil, the vault's long-term key is derived from it, and the
 // returned vault has it as its Mnemonic; when it is nil, the vault has no
-// long-term key until one is imported. When passphrase is not nil, the vault
-// gets a passphrase unlocker protected by it, as its current unlocker; that
-// needs a mnemonic. It refuses a vault that already exists before writing
-// anything: creating it again would replace its keys, and its secrets could
-// no longer be decrypted. The commands that call it hold the state directory
+// long-term key. When passphrase is not nil, the vault gets a passphrase
+// unlocker protected by it, as its current unlocker; that needs a mnemonic.
+// It refuses a vault that already exists before writing anything: creating
+// it again would replace its keys, and its secrets could no longer be
+// decrypted. The commands that call it hold the state directory
 // lock, so no other command can create the vault between the check and the
 // writes.
 //