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

@@ -240,11 +240,11 @@ func CheckNewVaultName(fs afero.Fs, stateDir string, name string) error {
 // 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 an invalid name or a vault that already exists,
-// as CheckNewVaultName does, before writing anything. The commands that call
-// it hold the state directory lock, so no other command can create the vault
+// 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 an invalid name or a vault that already exists, as
+// CheckNewVaultName does, before writing anything. The commands that call it
+// hold the state directory lock, so no other command can create the vault
 // between the check and the writes.
 //
 // The vault is written whole into a temporary directory, which is renamed