feat: cosmology encrypt subcommand; remove the legacy command alias

[pyramation]

Summary

dcrypt cosmology gains an encrypt subcommand, restoring the old CLI's ability to write the cosmology format (previously only decrypt/upgrade existed): plaintext from --in/stdin, salt from --salt-file/SALT/prompt, and the two-layer --encrypted-salt/ENCRYPTED_SALT path via encryptWithEncryptedSalt. Help text warns to prefer dcrypt encrypt for anything new.

Also removes the legacy command alias — dcrypt cosmology is the only name now.

Tests: old-format encrypt→decrypt round-trip through the CLI; the legacy-alias test is gone with the alias.

Link to Devin session: https://app.devin.ai/sessions/04636534e07048089ffb6b78142e12cd
Requested by: @pyramation

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring