fix: auto-upgrade marker no longer masks newer remote versions

When a just-upgraded-from marker persists across sessions, the update
check would write UP_TO_DATE to cache and exit immediately — never
fetching the remote VERSION. Users silently miss updates that landed
after their last upgrade.

Remove the early exit and premature cache write so the script falls
through to the remote check after consuming the marker. This ensures
JUST_UPGRADED is still emitted for the preamble, while also detecting
any newer versions available upstream.

Fixes #515

bin/gstack-update-check

@@ -113,12 +113,11 @@ if [ -f "$MARKER_FILE" ]; then
   OLD="$(cat "$MARKER_FILE" 2>/dev/null | tr -d '[:space:]')"
   rm -f "$MARKER_FILE"
   rm -f "$SNOOZE_FILE"
-  mkdir -p "$STATE_DIR"
-  echo "UP_TO_DATE $LOCAL" > "$CACHE_FILE"
   if [ -n "$OLD" ]; then
     echo "JUST_UPGRADED $OLD $LOCAL"
   fi
-  exit 0
+  # Don't exit — fall through to remote check in case
+  # more updates landed since the upgrade
 fi
 
 # ─── Step 3: Check cache freshness ──────────────────────────