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/vault/metadata_test.go

@@ -296,91 +296,6 @@ func TestSampleHashCalculation(t *testing.T) {
 	}
 }
 
-func TestWorkflowMismatch(t *testing.T) {
-	t.Parallel()
-
-	// Create a temporary directory for testing
-	tempDir := t.TempDir()
-	fs := afero.NewOsFs()
-
-	// Test Case 1: Create vault WITH mnemonic (like init command)
-	_, err := vault.CreateVault(fs, tempDir, "default", testMnemonicBuffer(t), nil)
-	if err != nil {
-		t.Fatalf("Failed to create vault with mnemonic: %v", err)
-	}
-
-	// Load metadata for vault1
-	vault1Dir := filepath.Join(tempDir, "vaults.d", "default")
-
-	metadata1, err := vault.LoadVaultMetadata(fs, vault1Dir)
-	if err != nil {
-		t.Fatalf("Failed to load vault1 metadata: %v", err)
-	}
-
-	t.Logf("Vault1 (with mnemonic) - DerivationIndex: %d, PublicKeyHash: %s",
-		metadata1.DerivationIndex, metadata1.PublicKeyHash)
-
-	// Test Case 2: Create vault WITHOUT mnemonic, then import (work vault)
-	_, err = vault.CreateVault(fs, tempDir, "work", nil, nil)
-	if err != nil {
-		t.Fatalf("Failed to create vault without mnemonic: %v", err)
-	}
-
-	vault2Dir := filepath.Join(tempDir, "vaults.d", "work")
-
-	// Simulate the vault import process: get the next available derivation
-	// index for this mnemonic
-	derivationIndex, err := vault.GetNextDerivationIndex(fs, tempDir, testMnemonic)
-	if err != nil {
-		t.Fatalf("Failed to get next derivation index: %v", err)
-	}
-
-	t.Logf("Next derivation index for import: %d", derivationIndex)
-
-	// Calculate public key hash from index 0 (same as in VaultImport)
-	identity0, err := agehd.DeriveIdentity(testMnemonic, 0)
-	if err != nil {
-		t.Fatalf("Failed to derive identity for index 0: %v", err)
-	}
-
-	publicKeyHash := vault.ComputeDoubleSHA256(
-		[]byte(identity0.Recipient().String()))
-
-	// Load existing metadata and update it (same as in VaultImport)
-	existingMetadata, err := vault.LoadVaultMetadata(fs, vault2Dir)
-	if err != nil {
-		t.Fatalf("Failed to load existing metadata: %v", err)
-	}
-
-	// Update metadata with new derivation info
-	existingMetadata.DerivationIndex = derivationIndex
-	existingMetadata.PublicKeyHash = publicKeyHash
-
-	err = vault.SaveVaultMetadata(fs, vault2Dir, existingMetadata)
-	if err != nil {
-		t.Fatalf("Failed to save vault metadata: %v", err)
-	}
-
-	// Load updated metadata for vault2
-	metadata2, err := vault.LoadVaultMetadata(fs, vault2Dir)
-	if err != nil {
-		t.Fatalf("Failed to load vault2 metadata: %v", err)
-	}
-
-	t.Logf("Vault2 (imported mnemonic) - DerivationIndex: %d, PublicKeyHash: %s",
-		metadata2.DerivationIndex, metadata2.PublicKeyHash)
-
-	// Verify that both vaults have the same public key hash
-	if metadata1.PublicKeyHash != metadata2.PublicKeyHash {
-		t.Errorf("Public key hashes don't match!")
-		t.Logf("Vault1 hash: %s", metadata1.PublicKeyHash)
-		t.Logf("Vault2 hash: %s", metadata2.PublicKeyHash)
-	} else {
-		t.Logf("SUCCESS: Both vaults have the same public key hash: %s",
-			metadata1.PublicKeyHash)
-	}
-}
-
 func TestReverseEngineerHash(t *testing.T) {
 	t.Parallel()