feat(insta-lab): card_writer 프롬프트에 글자수 가이드(오버플로우 예방)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

insta-lab/app/test_card_writer_prompt.py

@@ -0,0 +1,9 @@
+from app import card_writer
+
+
+def test_default_prompt_has_length_guidance():
+    p = card_writer.DEFAULT_PROMPT
+    # 글자수 가이드가 프롬프트에 포함됐는지
+    assert "22자" in p and "120자" in p
+    # 포맷 placeholder는 유지
+    assert "{category}" in p and "{keyword}" in p and "{articles}" in p