fix: complete repository audit

[Roburmaster]

Summary

This pull request fixes the issues found during the post-setup audit of the public OpenHeal repository.

Fixes

• align OpenHeal.toc with the repository's GPL-3.0-only license;
• remove stale MIT and conflicting-license statements from the changelog;
• fix false-positive merge-conflict detection in CI;
• verify both GPL files are identical and validate TOC license/version metadata;
• update actions/checkout to pinned v7.0.1;
• validate Lua and XML before creating releases;
• require release tags to match the TOC version;
• publish a SHA-256 checksum with each release archive;
• add monthly Dependabot updates for GitHub Actions.

No gameplay or addon runtime logic is changed.