Hold the encrypt and decrypt result in memory until it is written (closes #143)

With --output, secret encrypt and secret decrypt emptied that file before
reading any input, so a failed command left it empty, and encrypting a file
onto itself gave the encryption of nothing.

With --output they now hold the result in memory and open --output, with the
same call as before, only once encryption or decryption has succeeded. A
failed command leaves --output untouched, and --input can be the same file.
Without --output the result still goes to stdout as it is produced.

Model: opus-5-5

README.md

@@ -300,6 +300,9 @@ generates a new Age key.
 
 Decrypts data using an Age key stored as a secret.
 
+With `--output`, both hold the result in memory and write it to `--output` only
+once they have succeeded, so a failed command leaves `--output` untouched.
+
 ## Storage Architecture
 
 ### Directory Structure