docs(licence): README.md prose licence → CC-BY-SA-4.0

README.md is prose documentation; per estate policy (standards
LICENCE-POLICY.adoc Rule 1) prose is CC-BY-SA-4.0, not MPL-2.0. Owner-
authorised per-file header remediation (Addendum A2). Header line only;
no other content changed. (boj-server keeps README.md rather than .adoc
for Glama rendering — that exception is unchanged.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MpYSh6Wy8YMBH2E3qVyT7

Author: claude

README.md

@@ -1,5 +1,5 @@
 <!--
-SPDX-License-Identifier: MPL-2.0
+SPDX-License-Identifier: CC-BY-SA-4.0
 SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 -->