The age commands: pub, priv, encrypt and decrypt (closes #3)

The application number is 657169, so the path is
m/83696968'/657169'/<n>'. The 32 derived bytes are clamped the way
X25519 requires and go through bech32 into an age identity, the only
route age offers from raw bytes to a key; these are the steps
sneak/secret takes in its agehd package. A test fixes the secret key
the example mnemonic gives at index 0, so a change to any of those
steps is caught.

The derived recipient is always first, so the mnemonic that encrypted
a file can read it back. Decrypting recognises the text form by its
first line, so it needs no flag. A file named with -o is written
beside the target, readable only by its owner, and renamed into place
once the work succeeds, so a refused decryption leaves what was
already there untouched.

Model: opus-5

internal/cli/cli.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 
+	"git.eeqj.de/sneak/keyfunc/internal/cli/age"
 	"git.eeqj.de/sneak/keyfunc/internal/cli/mnemonic"
 	"git.eeqj.de/sneak/keyfunc/internal/cli/options"
 	"git.eeqj.de/sneak/keyfunc/internal/cli/ssh"
@@ -30,7 +31,7 @@ func Root() *cobra.Command {
 	}
 
 	options.Add(root)
-	root.AddCommand(ssh.Command(), mnemonic.Command())
+	root.AddCommand(ssh.Command(), age.Command(), mnemonic.Command())
 
 	return root
 }