docs: add "not a deployed clinical device" disclaimer near top

Per codex audit: existing README has hackathon framing in three
places (badge, line 16, limitations on line 131), but the explicit
"not deployed / regulatory work in progress" signal is only at
line 131. For clinical-AI roles a recruiter scanning the first
screen needs that disclaimer up top, not buried.

One italic line directly after the existing hackathon mention,
above the dashboard screenshot. Wording kept light per the brief.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Ekin-Kahraman

README.md

@@ -15,6 +15,8 @@ SafetyNett closes that gap. GPs create condition-specific safety nets at the poi
 
 Built in 2.5 hours at the [OpenClaw Clinical Hackathon](https://clinicalhackathon.com) (28 March 2026, Accurx HQ, London).
 
+*Hackathon prototype, not a deployed clinical device.*
+
 ![SafetyNett Dashboard](screenshot.png)
 
 ## Architecture