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/CLAUDE.md

@@ -12,6 +12,8 @@ python3 scripts/gsc_signal_delta.py \
 Returns day-normalized site totals, top gainers/decliners, and a `verdict_hint`
 (heuristic only — the final verdict is the skill's job, after L2/L3).
 
+**Surge-tuning note**: `verdict_hint` and `in_top_movers` are calibrated for upward "surge" claims (movers ranked by click gain). For a claimed *drop*, inspect `top_decliners` directly rather than relying on the hint.
+
 ## Getting the exports
 `mcp__dda__gsc_fetch_performance` (property pinned per workspace, e.g. JHR
 `sc-domain:josunhotel.com`) → save the query-dimension rows to a file → run the