Add GPL-3.0 license, PCM packaging, and cross-platform deploy

- LICENSE.txt (GPL-3.0-or-later), declared in the new PCM v2
  metadata.json (validated against the official schema)
- tools/build_package.py builds the registry-layout addon zip in dist/
  plus the submission metadata copy with sha256/sizes and a release
  download_url derived from the Gitea homepage
- tools/deploy.py: symlink/copy deploy for Linux, macOS and Windows
- 64 px resources/icon.png for the PCM listing
- README: Linux setup/dev commands, packaging and license sections

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -2,3 +2,4 @@
 __pycache__/
 *.pyc
 .pytest_cache/
+dist/