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/unreadable_dir_test.go

@@ -371,19 +371,3 @@ func TestRemoveSecretAbortsWhenVersionsUnreadable(t *testing.T) {
 	require.NoError(t, err)
 	assert.True(t, exists, "the secret must not be removed")
 }
-
-// TestVaultImportAbortsWhenPubKeyUnreadable asserts that a mnemonic import
-// stops when whether the vault already has a long-term key cannot be
-// determined.
-func TestVaultImportAbortsWhenPubKeyUnreadable(t *testing.T) {
-	t.Parallel()
-
-	base := newListTestVault(t, 1)
-	instance, cmd := newTestInstance(&statFailFs{
-		Fs: base, path: filepath.Join(testVaultDir(listTestVaultName), "pub.age"),
-	})
-
-	err := instance.importMnemonic(cmd, listTestVaultName)
-
-	require.ErrorIs(t, err, errStatFailed)
-}