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

CONTRIBUTING.md

@@ -30,10 +30,12 @@ release builds.
 
 ## Conventions
 
+<!-- REUSE-IgnoreStart -->
 - Every source file starts with `SPDX-License-Identifier: GPL-3.0-or-later`
   (in whatever comment syntax fits the file type). Files that can't carry
   an inline header — `.desktop`, `keys/*.asc` — are covered instead by
   `REUSE.toml`.
+<!-- REUSE-IgnoreEnd -->
 - `src/` is intentionally flat: one class per concern, no `models/`,
   `controllers/`, or `viewmodels/` subfolders.
 - C++ backend classes are exposed to QML via `QML_ELEMENT`; QML views are