chore(spdx): normalize headers (code -> MPL-2.0, prose -> CC-BY-4.0) (#150)

## Summary

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

| Extension | License |
|-----------|---------|
| `*.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.

## Background

A previous session attempted this normalization but the changes did not
persist due to cross-session branch flipping interference. This PR was
prepared in a fresh worktree off `origin/main` with a single atomic
commit to avoid that failure mode.

## Test plan

- [ ] CI passes (license-consistency, agda-build)
- [ ] Spot-check a few files render correctly with the new headers
- [ ] No SPDX-header lint complaints

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

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