docs(readme): add SPDX header and license badge

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath

README.adoc

@@ -1,8 +1,12 @@
+// SPDX-License-Identifier: PMPL-1.0-or-later
+// SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+
 = boj-server
 :toc: preamble
 :icons: font
 
 image:https://img.shields.io/badge/OpenSSF-Best_Practices-green?logo=opensourcesecurity[OpenSSF Best Practices,link="https://www.bestpractices.dev/en/projects/new?repo_url=https://github.com/hyperpolymath/boj-server"]
+image:https://img.shields.io/badge/License-PMPL--1.0-blue.svg[License: PMPL-1.0,link="https://github.com/hyperpolymath/palimpsest-license"]
 image:https://glama.ai/mcp/servers/hyperpolymath/boj-server/badge[Glama MCP Server,link=https://glama.ai/mcp/servers/hyperpolymath/boj-server]
 image:https://api.thegreenwebfoundation.org/greencheckimage/boj-server.net[Green Hosting,link="https://www.thegreenwebfoundation.org/green-web-check/?url=boj-server.net"]