fix(ci): check-spdx prose licence variable → CC-BY-SA-4.0

check-spdx.sh set the prose SPDX variable to MPL-2.0, which contradicted the
repo's own machine-readable declaration (licensing-policy.toml: prose =
CC-BY-SA-4.0) and the checker's own failure label. Every prose file already
carries CC-BY-SA-4.0, so the checker was failing `just license-check` on a
pristine tree. Point the variable at the declared policy and exclude the
canonical LICENSES/ REUSE texts (mirrored in licensing-policy.toml).

No licence header on any file is changed — this only corrects the enforcer to
accept the headers that are already present and declared.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MpYSh6Wy8YMBH2E3qVyT7

Author: claude

.machine_readable/licensing-policy.toml

@@ -22,6 +22,7 @@ prose = "CC-BY-SA-4.0"  # *.adoc, *.md narrative
 paths = [
   "Cargo.lock",      # generated by cargo; enumerates third-party dependencies
   "LICENSE",         # the MPL-2.0 licence text itself
+  "LICENSES/",       # canonical SPDX licence texts (REUSE dir: MPL-2.0 + CC-BY-SA-4.0)
   "target/",         # cargo build output (gitignored)
   "tests/fixtures/", # Agda test-input data (some deliberately malformed)
 ]

scripts/check-spdx.sh

@@ -5,7 +5,7 @@
 # Licence invariant for arghda-core (enforced by `just check` + Rust CI).
 #
 #   code / config / scripts / state   ->  SPDX-License-Identifier: MPL-2.0
-#   prose documentation (*.adoc, *.md) ->  SPDX-License-Identifier: MPL-2.0
+#   prose documentation (*.adoc, *.md) ->  SPDX-License-Identifier: CC-BY-SA-4.0
 #
 # Files that are NOT ours, generated, or test-input data are EXCLUDED and must
 # never carry the repo's licence choices. The machine-readable declaration of
@@ -20,13 +20,14 @@ set -euo pipefail
 cd "$(git rev-parse --show-toplevel)"
 
 MPL='SPDX-License-Identifier: MPL-2.0'
-CC='SPDX-License-Identifier: MPL-2.0'
+CC='SPDX-License-Identifier: CC-BY-SA-4.0'
 fail=0
 
 # Not ours / generated / test-input data — see [excluded] in the policy file.
 excluded() {
   case "$1" in
     Cargo.lock | LICENSE) return 0 ;;          # generated lockfile / the licence text itself
+    LICENSES/*) return 0 ;;                     # canonical SPDX licence texts (REUSE dir)
     target/* | tests/fixtures/*) return 0 ;;   # build output / Agda test-input data
     *) return 1 ;;
   esac