Read secret environment variables once per command, then unset them (closes #60)

init and vault create put the mnemonic into the process environment for
vault.CreateVault to read back, so every program they ran, gpg included,
inherited it, and SB_SECRET_MNEMONIC and SB_UNLOCK_PASSPHRASE were read
at 13 places and never unset. Each command that may need them now reads
both once, in its RunE, into locked buffers on the CLI Instance, and
unsets them at once. The buffers are passed down: vault.CreateVault
takes the mnemonic, a Vault carries Mnemonic and UnlockPassphrase, and
the PGP, keychain and Secure Enclave unlocker constructors take both;
CreatePGPUnlocker sets them on the vault it loads through SetMnemonic
and SetUnlockPassphrase, new in VaultInterface. README warns against
both variables.

Model: opus-5-5

internal/cli/integration_test.go

@@ -286,7 +286,7 @@ func TestSecretManagerIntegration(t *testing.T) {
 	// Test 25: Concurrent operations
 	// Purpose: Test multiple simultaneous operations
 	// Expected: Proper locking/synchronization, no corruption
-	test25ConcurrentOperations(t, testMnemonic, runSecret, runSecretWithEnv)
+	test25ConcurrentOperations(t, tempDir, secretPath, testMnemonic, runSecret)
 
 	// Test 26: Large secret values
 	// Purpose: Test with large secret values (e.g., certificates)
@@ -2009,28 +2009,35 @@ func test24EnvironmentVariables(t *testing.T, tempDir, secretPath, testMnemonic,
 	assert.Equal(t, "env-test-value", strings.TrimSpace(string(cmdOutput2)))
 }
 
-func test25ConcurrentOperations(t *testing.T, testMnemonic string, runSecret func(...string) (string, error), runSecretWithEnv func(map[string]string, ...string) (string, error)) {
+func test25ConcurrentOperations(t *testing.T, tempDir, secretPath, testMnemonic string, runSecret func(...string) (string, error)) {
 	t.Helper()
 
 	// Make sure we're in default vault
 	_, err := runSecret("vault", "select", "default")
 	require.NoError(t, err, "vault select should succeed")
 
-	// Run multiple concurrent reads
+	// Run multiple concurrent reads, as separate processes: within one
+	// process the first command to read the mnemonic would unset it for
+	// the others
 	const numReaders = 5
 
 	errCh := make(chan error, numReaders)
 
 	for i := range numReaders {
 		go func(id int) {
-			output, err := runSecretWithEnv(map[string]string{
-				secret.EnvMnemonic: testMnemonic,
-			}, "get", "database/password")
+			cmd := exec.CommandContext(t.Context(), secretPath, "get", "database/password")
+			cmd.Env = []string{
+				secret.EnvStateDir + "=" + tempDir,
+				secret.EnvMnemonic + "=" + testMnemonic,
+				"PATH=" + os.Getenv("PATH"),
+				"HOME=" + os.Getenv("HOME"),
+			}
+			output, err := cmd.Output()
 
 			switch {
 			case err != nil:
 				errCh <- fmt.Errorf("reader %d failed: %w", id, err)
-			case strings.TrimSpace(output) == "":
+			case strings.TrimSpace(string(output)) == "":
 				errCh <- fmt.Errorf("%w: reader %d", errEmptyValue, id)
 			default:
 				errCh <- nil