fix(skill): absent claim term -> INCONCLUSIVE hint; note surge-tuning (final-review minors #3,#4)

- gsc_signal_delta.py: extract `found` local var; add first branch in verdict_hint
  chain so a term absent from both GSC windows yields INCONCLUSIVE (not ARTIFACT).
  Existing ARTIFACT / CONFIRMED-PARTIAL / PARTIAL branches unchanged (elif chain).
- test_gsc_signal_delta.py: add test_absent_claim_term_inconclusive asserting
  found=False and "INCONCLUSIVE" in verdict_hint for a term in neither fixture.
- code/CLAUDE.md: one-line surge-tuning note — verdict_hint/in_top_movers are
  calibrated for upward claims; for drops, inspect top_decliners directly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KuT3W81t88QQFaxY2ruWv2

custom-skills/35-seo-signal-validation/code/scripts/test_gsc_signal_delta.py

@@ -40,6 +40,12 @@ def test_day_normalization():
     assert out["site_totals"]["prior"]["clicks_per_day"] == 6.93     # 208/30
 
 
+def test_absent_claim_term_inconclusive():
+    out = compute_delta(RECENT, PRIOR, 28, 30, claim_term="존재하지않는검색어")
+    assert out["claim_term"]["found"] is False
+    assert "INCONCLUSIVE" in out["verdict_hint"]
+
+
 def test_positive_days_required():
     try:
         compute_delta(RECENT, PRIOR, 0, 30)