ax(node): replace prose constant comment with usage example

ChallengeSize comment restated the name ("is the size of the challenge
in bytes") — violating AX Principle 2. Replace with a concrete usage
example showing how the constant is used at call sites.

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/identity.go

@@ -18,7 +18,8 @@ import (
 	"github.com/adrg/xdg"
 )
 
-// ChallengeSize is the size of the challenge in bytes
+// challenge := make([]byte, ChallengeSize)
+// rand.Read(challenge)
 const ChallengeSize = 32
 
 // challenge, err := node.GenerateChallenge()