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/lock_test.go

@@ -282,11 +282,10 @@ func stateDirModTimes(t *testing.T, fs afero.Fs) map[string]int64 {
 
 // setupEveryCommand makes what each command in
 // TestChangingCommandsWaitForLock needs: the current vault "work" with two
-// versions of "test/secret", the vault "other" without a long-term key, for
-// vault import, and the file testInput. There is no vault "default", which
-// init creates. If withUnlocker is set, it also gives "work" a passphrase
-// unlocker, which is slow. It returns the older version and the unlocker's
-// ID.
+// versions of "test/secret", the vault "other", and the file testInput.
+// There is no vault "default", which init creates. If withUnlocker is set,
+// it also gives "work" a passphrase unlocker, which is slow. It returns the
+// older version and the unlocker's ID.
 func setupEveryCommand(
 	t *testing.T, fs afero.Fs, withUnlocker bool,
 ) (string, string) {
@@ -294,13 +293,9 @@ func setupEveryCommand(
 
 	mnemonic := testMnemonicBuffer(t)
 
-	other, err := vault.CreateVault(fs, testStateDir, "other", mnemonic, nil)
+	_, err := vault.CreateVault(fs, testStateDir, "other", mnemonic, nil)
 	require.NoError(t, err)
 
-	otherDir, err := other.GetDirectory()
-	require.NoError(t, err)
-	require.NoError(t, fs.Remove(filepath.Join(otherDir, "pub.age")))
-
 	vlt, err := vault.CreateVault(fs, testStateDir, "work", mnemonic, nil)
 	require.NoError(t, err)
 
@@ -455,9 +450,6 @@ func TestChangingCommandsWaitForLock(t *testing.T) {
 		{"vault select", false, func(cli *Instance, _, _ string) error {
 			return cli.SelectVault(cli.cmd, "other")
 		}},
-		{"vault import", false, func(cli *Instance, _, _ string) error {
-			return cli.VaultImport(cli.cmd, "other")
-		}},
 		{"vault rm --force", false, func(cli *Instance, _, _ string) error {
 			return cli.RemoveVault(cli.cmd, "other", true)
 		}},