chore: establish GPL-3.0 open-source baseline

[Roburmaster]

Summary

This pull request converts OpenHeal from MIT to GPL-3.0-only and establishes a maintainable public open-source repository baseline.

Included

• canonical GPL-3.0 license in the repository root and packaged addon;
• project README with installation, development, support, contribution, release, and trademark guidance;
• security policy and private vulnerability reporting instructions;
• contribution guide and project code of conduct;
• CODEOWNERS and structured pull request review checklist;
• bug and feature issue forms with secure-frame/WoW-specific diagnostics;
• .editorconfig, .gitattributes, and .gitignore;
• CI validation for Lua 5.1 syntax, XML syntax, required files, merge markers, and TOC references;
• automated installable zip and GitHub Release creation for v*.*.* tags.

Important repository settings after merge

GitHub settings that are not stored in the repository should be enabled separately: branch protection/ruleset for main, private vulnerability reporting, secret scanning and push protection where available, deletion of head branches after merge, and squash merging.

Validation

The validation workflow will run against this pull request. The one-time workflow used to install the canonical GPL text was removed after both license files were committed.