fix(ci): add SPDX-License-Identifier header to LICENSE (#284)

Fixes the pre-existing **Governance → "Licence consistency"** failure on
`main`.

`standards`' `check-licence-consistency.sh` scans the caller repo's
`LICENSE` for an `SPDX-License-Identifier:` header on its first lines
and exits 1 if absent. echo-types' `LICENSE` is the verbatim MPL-2.0
text with no SPDX line, so the gate fails.

This prepends the matching identifier + copyright:
```
SPDX-License-Identifier: MPL-2.0
SPDX-FileCopyrightText: 2024-2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
```
No relicensing — the file is already MPL-2.0; this adds the
machine-readable identifier the gate requires. Sole-owner repo → MPL-2.0
per estate policy.

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

https://claude.ai/code/session_019awZjBD1qx61tvmEuEKNpn

---
_Generated by [Claude
Code](https://claude.ai/code/session_019awZjBD1qx61tvmEuEKNpn)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

LICENSE

@@ -1,3 +1,6 @@
+SPDX-License-Identifier: MPL-2.0
+SPDX-FileCopyrightText: 2024-2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+
 Mozilla Public License Version 2.0
 ==================================