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/create_vault_test.go

@@ -141,6 +141,30 @@ func TestVaultCreationLeavesNoSecretInEnvironment(t *testing.T) {
 	run("vault", "create", "work")
 }
 
+// TestNoVaultImportCommand is a regression test for
+// https://git.eeqj.de/sneak/secret/issues/139: `secret vault import`
+// refused every vault that `secret vault create` makes, because each has a
+// long-term key. The command is gone, so cobra answers it with the help of
+// `secret vault`, as it does any subcommand of `secret vault` it does not
+// know.
+func TestNoVaultImportCommand(t *testing.T) {
+	t.Setenv(secret.EnvStateDir, t.TempDir())
+
+	run := func(args ...string) string {
+		t.Setenv(secret.EnvMnemonic, testMnemonic)
+		t.Setenv(secret.EnvUnlockPassphrase, testPassphrase)
+
+		output, err := cli.ExecuteCommandInProcess(args, "", nil)
+		require.NoError(t, err, "%v", args)
+
+		return output
+	}
+
+	run("vault", "create", "work")
+
+	require.Equal(t, run("vault"), run("vault", "import", "work"))
+}
+
 // TestStopAtPassphrasePromptLeavesNothing is a regression test for the
 // review of https://git.eeqj.de/sneak/secret/pulls/82: `secret init` or
 // `secret vault create` stopped at the passphrase prompt left a vault with