docs: dual-license GOVERNANCE/MAINTAINERS (MPL-2.0 + CC-BY-SA-4.0) (#35)

Follow-up to #34. Adds a second SPDX line — `CC-BY-SA-4.0` — to the
`GOVERNANCE.adoc` and `MAINTAINERS.adoc` headers, alongside the existing
`MPL-2.0`.

- **MPL-2.0** stays listed first and remains the repo's primary/winning
licence (root `LICENSE` is MPL-2.0).
- **CC-BY-SA-4.0** covers the prose content, per the estate docs-licence
policy.

Purely additive (one line per file). Commit signed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

GOVERNANCE.adoc

@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: MPL-2.0
+// SPDX-License-Identifier: CC-BY-SA-4.0
 // Copyright (c) Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 = Governance
 

MAINTAINERS.adoc

@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: MPL-2.0
+// SPDX-License-Identifier: CC-BY-SA-4.0
 // Copyright (c) Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 = Maintainers