fix: update check ignores stale UP_TO_DATE cache after version change

The UP_TO_DATE cache path exited immediately without checking if the
cached version still matched the local VERSION. After upgrading (e.g.
0.3.3 → 0.3.4), the cache still said "UP_TO_DATE 0.3.3" and the
script never re-checked against remote — so updates were invisible
until the 24h cache expired.

Now both UP_TO_DATE and UPGRADE_AVAILABLE verify cached version vs
local before trusting the cache.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bin/gstack-update-check

@@ -49,7 +49,12 @@ if [ -f "$CACHE_FILE" ]; then
     CACHED="$(cat "$CACHE_FILE" 2>/dev/null || true)"
     case "$CACHED" in
       UP_TO_DATE*)
-        exit 0
+        # Verify local version still matches cached version
+        CACHED_VER="$(echo "$CACHED" | awk '{print $2}')"
+        if [ "$CACHED_VER" = "$LOCAL" ]; then
+          exit 0
+        fi
+        # Local version changed — fall through to re-check
         ;;
       UPGRADE_AVAILABLE*)
         # Verify local version still matches cached old version