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
failure to encrypt or decrypt leaves --output untouched, and --input can be the
same file. A failure while writing --output, such as a full disk, can leave it
empty or partly written; an error from closing it is reported. Without
--output the result still goes to stdout as it is produced.

Model: opus-5-5

README.md

@@ -314,6 +314,12 @@ name. The file's bytes are stored unchanged, a final newline included.
 
 ### Encryption Operations
 
+With `--output`, `secret encrypt` and `secret decrypt` hold the result in memory
+and write it to `--output` only once encryption or decryption has succeeded, so
+a failure to encrypt or decrypt leaves `--output` untouched. A failure while
+writing `--output`, such as a full disk, can leave it empty or partly written.
+The command needs about as much memory as the result.
+
 #### `secret encrypt <secret-name> [--input=file] [--output=file]`
 
 Encrypts data using an Age key stored as a secret. If the secret doesn't exist,