Allow keeping encrypted version

[geek-merlin]

When working in a development site, sometimes i'm decrypting my secrets (with gitignore ensuring that they do not get committed).

But then the encrypted file is deleted, and there is no simple way to prevent this. If that slips through into a commit, my deployments are broken.

# What would help my workflow:
# Opt into no delete on decrypt.
agebox init --keep-encrypted
...
agebox decrypt --all
# Encrypted files are still around.