fix(ci): add missing SPDX headers to workflow files

The Governance reusable's "Workflow security linter" job requires every
.github/workflows/*.yml to carry an SPDX-License-Identifier on line 1. These
files were missing it, so the Governance workflow failed on every push — one
more red workflow feeding the ci_activity notification flood.

Identifier (MPL-2.0) is taken from this repo's own existing workflow headers,
which agree unanimously — not assumed. Files: pages.yml

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/pages.yml

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: MPL-2.0
 name: GitHub Pages (Ddraig SSG)
 on:
   push: