ci: build add-on packages in GitHub Actions

[bee-san]

Summary

• Add a CI workflow that runs unit tests on Python 3.10, 3.11, and 3.12.
• Add CI packaging that builds and uploads anki_vn_sorter.ankiaddon as an artifact.
• Add a tag/manual release workflow that builds, verifies, and publishes the .ankiaddon release asset plus SHA-256 file.
• Update the README install section to direct users to GitHub Releases instead of local builds.

Test plan

• actionlint .github/workflows/ci.yml .github/workflows/release.yml
• Ruby YAML parse of both workflows
• bash -n on all workflow run: blocks after GitHub expression substitution
• python3.11 -m unittest discover -s tests
• python3.11 -m compileall addon scripts tests
• python3.11 scripts/package_addon.py
• Local package inspection: required files present; no bytecode/user files included
• HEAD request for releases/latest/download/anki_vn_sorter.ankiaddon resolves to the existing release asset

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud