fix: exclude gnu.org and fsf.org from link checking

These domains rate-limit GitHub Actions CI runners (returning 429),
causing spurious link check failures. They are well-known stable URLs
that don't need verification.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tobixen

.lycheeignore

@@ -0,0 +1,4 @@
+# gnu.org and fsf.org rate-limit CI runners (returns 429), but these are
+# well-known stable URLs that don't need verification.
+https://www\.gnu\.org/licenses/.*
+https://fsf\.org/