Keep secret get values in locked memory (closes #37)

Vault.GetSecret and Vault.GetSecretVersion return the decrypted value
as a *memguard.LockedBuffer instead of copying it into an ordinary
[]byte that nothing wiped. Every caller destroys the buffer, and
`secret get` writes its bytes straight to stdout, still with no
trailing newline. Instance.Print, which formatted through fmt and had
no other callers, is removed, and so is a debug log line in
`get --version` that held the plaintext value.

Model: opus-5-5

internal/cli/lock_test.go

@@ -147,7 +147,8 @@ func TestConcurrentAddsKeepEveryVersion(t *testing.T) {
 				value, err := vlt.GetSecretVersion("shared", version)
 				require.NoError(t, err)
 
-				values[string(value)] = true
+				values[string(value.Bytes())] = true
+				value.Destroy()
 			}
 
 			assert.Len(t, values, adds+1, "every add stored its own value")