Fix age-keygen hanging due to pipe buffering

Extract public key from generated file instead of parsing
stderr through a pipe, which could cause blocking.

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

setup

@@ -80,8 +80,9 @@ setup_new() {
     else
         if has age-keygen; then
             step "Generating age encryption key..."
-            age-keygen -o "$AGE_KEY_FILE" 2>&1 | grep -i "public key"
+            age-keygen -o "$AGE_KEY_FILE" 2>/dev/null
             chmod 600 "$AGE_KEY_FILE"
+            grep '^# public key:' "$AGE_KEY_FILE" | sed 's/^# //'
         else
             err "age not installed. Run: nix-shell -p age"
             exit 1