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/integration_test.go

@@ -140,16 +140,6 @@ func TestSecretManagerIntegration(t *testing.T) {
 	//   - currentvault symlink updated to point to work
 	test03CreateVault(t, tempDir, runSecret)
 
-	// Test 4: Import mnemonic into work vault
-	// Command: secret vault import work
-	// Purpose: Import mnemonic with passphrase unlocker
-	// Expected filesystem:
-	//   - work vault has pub.age file
-	//   - work vault has unlockers.d/passphrase directory
-	//   - Unlocker metadata and encrypted keys present
-	// NOTE: Skipped because vault creation now includes mnemonic import
-	// test04ImportMnemonic(t, tempDir, testMnemonic, testPassphrase, runSecretWithEnv)
-
 	// Test 5: Add secrets with versioning
 	// Command: echo "password123" | secret add database/password
 	// Purpose: Add a secret and verify versioned storage
@@ -535,65 +525,6 @@ func test03CreateVault(t *testing.T, tempDir string, runSecret func(...string) (
 	assert.Contains(t, output, "work", "should list work vault")
 }
 
-//nolint:unused // TODO: re-enable when vault import is implemented
-func test04ImportMnemonic(t *testing.T, tempDir, testMnemonic, testPassphrase string, runSecretWithEnv func(map[string]string, ...string) (string, error)) {
-	t.Helper()
-
-	// Import mnemonic into work vault
-	output, err := runSecretWithEnv(map[string]string{
-		secret.EnvMnemonic:         testMnemonic,
-		secret.EnvUnlockPassphrase: testPassphrase,
-	}, "vault", "import", "work")
-
-	require.NoError(t, err, "vault import should succeed")
-	assert.Contains(t, output, "Successfully imported mnemonic into vault 'work'", "should confirm import")
-
-	// Verify work vault now has long-term key
-	workVaultDir := filepath.Join(tempDir, "vaults.d", "work")
-	pubKeyFile := filepath.Join(workVaultDir, "pub.age")
-	verifyFileExists(t, pubKeyFile)
-
-	// Verify passphrase unlocker was created
-	currentUnlocker := readFile(t, filepath.Join(workVaultDir, "current-unlocker"))
-	passphraseUnlockerDir := filepath.Join(workVaultDir, "unlockers.d", string(currentUnlocker))
-	verifyFileExists(t, passphraseUnlockerDir)
-
-	// Check unlocker files
-	unlockerMetadata := filepath.Join(passphraseUnlockerDir, "unlocker-metadata.json")
-	verifyFileExists(t, unlockerMetadata)
-
-	encryptedLTPrivKey := filepath.Join(passphraseUnlockerDir, "priv.age")
-	verifyFileExists(t, encryptedLTPrivKey)
-
-	encryptedLTPubKey := filepath.Join(passphraseUnlockerDir, "pub.age")
-	verifyFileExists(t, encryptedLTPubKey)
-
-	longtermKeyFile := filepath.Join(passphraseUnlockerDir, "longterm.age")
-	verifyFileExists(t, longtermKeyFile)
-
-	// Verify vault metadata was created with derivation index
-	vaultMetadata := filepath.Join(workVaultDir, "vault-metadata.json")
-	verifyFileExists(t, vaultMetadata)
-
-	metadataBytes := readFile(t, vaultMetadata)
-
-	var metadata map[string]any
-
-	err = json.Unmarshal(metadataBytes, &metadata)
-	require.NoError(t, err, "vault metadata should be valid JSON")
-
-	// Work vault should have a different derivation index than default (0)
-	derivIndex, ok := metadata["derivationIndex"].(float64)
-	require.True(t, ok, "derivationIndex should be a number")
-	assert.NotEqual(t, float64(0), derivIndex, "work vault should have non-zero derivation index")
-
-	// Verify public key hash is stored
-	assert.Contains(t, metadata, "publicKeyHash", "should contain public key hash")
-	pubKeyHash, ok := metadata["publicKeyHash"].(string)
-	require.True(t, ok, "publicKeyHash should be a string")
-	assert.NotEmpty(t, pubKeyHash, "public key hash should not be empty")
-}
-
 func test05AddSecret(t *testing.T, tempDir, testMnemonic string, runSecret func(...string) (string, error), runSecretWithEnv func(map[string]string, ...string) (string, error), runSecretWithStdin func(string, map[string]string, ...string) (string, error)) {
 	t.Helper()
 
@@ -1939,14 +1870,6 @@ func test23ErrorHandling(t *testing.T, tempDir, secretPath, testMnemonic string,
 	_, err = runSecret("vault", "select", "nonexistent")
 	require.ErrorIs(t, err, vault.ErrVaultNotFound, "select non-existent vault should fail")
 
-	// Import to non-existent vault with test passphrase
-	testPassphrase := "test-passphrase-123" // Define testPassphrase locally
-	_, err = runSecretWithEnv(map[string]string{
-		secret.EnvMnemonic:         testMnemonic,
-		secret.EnvUnlockPassphrase: testPassphrase,
-	}, "vault", "import", "nonexistent")
-	require.ErrorIs(t, err, vault.ErrVaultNotFound, "import to non-existent vault should fail")
-
 	// Get specific version that doesn't exist
 	_, err = runSecretWithEnv(map[string]string{
 		secret.EnvMnemonic: testMnemonic,