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

LICENSES/MIT.txt

@@ -0,0 +1,18 @@
+MIT License
+
+Copyright (c) <year> <copyright holders>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and
+associated documentation files (the "Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the
+following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial
+portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
+LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO
+EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+USE OR OTHER DEALINGS IN THE SOFTWARE.