chore(spdx): normalize headers (code -> MPL-2.0, prose -> CC-BY-4.0)

One-shot SPDX-header normalization across the repo, applying the canonical
licensing convention documented in README §Licensing.

- 261 files received new SPDX headers (no prior header present)
- 14 files had their SPDX-License-Identifier corrected:
  * 11 prose files (.adoc / .md): MPL-2.0 -> CC-BY-4.0
  * 3 prose files: PMPL-1.0-or-later -> CC-BY-4.0 (stragglers missed
    by the 97e9666 PMPL-1.0-or-later -> MPL-2.0 migration)

Mapping per file extension:
- *.agda, *.sh, *.toml, *.nix, *.yml, *.yaml, Justfile, Containerfile -> MPL-2.0
- *.md, *.adoc, *.cff -> CC-BY-4.0

Comment style picked per extension; SPDX block placed after the OPTIONS
pragma on `.agda` files and after the shebang on `.sh` files. Existing
identifiers are replaced in place to preserve the comment shell.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

CHANGELOG.md

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: CC-BY-4.0
+# SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 cff-version: 1.2.0
 title: "echo-types: a definitionally-grounded, loss-graded reindexing modality (Agda)"
 message: "If you cite this software, please cite the Zenodo DOI."

CITATION.cff

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: MPL-2.0
+# SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 # Echo Types Build and Test System
 
 # Build all echo type modules