Fix the Lint CI workflow (broken since v0.1.2)

clang-format job: the flatpak CI image has no ECM on the host, so the
configure step that generated .clang-format could never run. Commit the
ECM-generated .clang-format instead (it is MIT, LICENSES/MIT.txt added),
drop the container, pin clang-format 22.1.8 from PyPI to match the
version the tree is formatted with, and reformat the sources to match.

REUSE job: add the missing CC0-1.0 license text, a copyright line for
the metainfo, REUSE.toml coverage for README/CONTRIBUTING/.gitignore,
and REUSE-Ignore markers around CONTRIBUTING.md's SPDX example so the
parser stops reading prose as a license declaration. reuse lint now
passes locally (54/54 files).

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019FXfh48mASRsE1WEhy9jcM

.github/workflows/lint.yml

@@ -22,15 +22,16 @@ jobs:
   clang-format:
     name: clang-format
     runs-on: ubuntu-latest
-    container:
-      image: ghcr.io/flathub-infra/flatpak-github-actions:kde-6.10
-      options: --privileged
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Configure (generates the KDE .clang-format style)
-        run: cmake -B build -G Ninja -DBUILD_TESTING=OFF
+      # The .clang-format at the repo root is the one ECM's KDEClangFormat
+      # generates at configure time, committed so this check needs no KDE
+      # stack. Pinned to the clang-format major used to format the tree -
+      # output drifts between major versions.
+      - name: Install clang-format
+        run: pipx install clang-format==22.1.8
 
       - name: Check formatting
         run: |