Fix case-sensitive grep breaking age key generation

age-keygen outputs "Public key:" but grep was searching for
lowercase "public key", causing silent exit with pipefail.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

setup

@@ -80,7 +80,7 @@ setup_new() {
     else
         if has age-keygen; then
             step "Generating age encryption key..."
-            age-keygen -o "$AGE_KEY_FILE" 2>&1 | grep "public key"
+            age-keygen -o "$AGE_KEY_FILE" 2>&1 | grep -i "public key"
             chmod 600 "$AGE_KEY_FILE"
         else
             err "age not installed. Run: nix-shell -p age"