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/cli/input_errors_test.go

@@ -7,12 +7,10 @@ import (
 	"github.com/awnumar/memguard"
 	"github.com/spf13/afero"
 	"github.com/stretchr/testify/require"
-	"sneak.berlin/go/secret/internal/vault"
 )
 
 // TestInvalidMnemonicError checks that every command that takes a mnemonic
-// returns errInvalidMnemonicPhrase for one that is not valid BIP39. The vault
-// "other" has no long-term key, as vault import needs.
+// returns errInvalidMnemonicPhrase for one that is not valid BIP39.
 func TestInvalidMnemonicError(t *testing.T) {
 	t.Parallel()
 
@@ -24,20 +22,13 @@ func TestInvalidMnemonicError(t *testing.T) {
 		{"secret vault create work", func(c *Instance) error {
 			return c.CreateVault(c.cmd, "work")
 		}},
-		{"secret vault import other", func(c *Instance) error {
-			return c.VaultImport(c.cmd, "other")
-		}},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.command, func(t *testing.T) {
 			t.Parallel()
 
-			fs := afero.NewMemMapFs()
-			_, err := vault.CreateVault(fs, listTestStateDir, "other", nil, nil)
-			require.NoError(t, err)
-
-			instance, _ := newTestInstance(fs)
+			instance, _ := newTestInstance(afero.NewMemMapFs())
 			instance.Mnemonic = memguard.NewBufferFromBytes([]byte("not a mnemonic"))
 			t.Cleanup(instance.Mnemonic.Destroy)