docs(project): prepare repository for open source

[Saba-Burduli]

Summary

• Prepare Petty for public open-source collaboration.
• Clarify the license boundary between original source code and bundled third-party assets.
• Add contributor, security, conduct, issue, pull-request, and CI documentation.
• Make AppKit controllers explicitly main-actor isolated for Xcode 15 CI compatibility.

Changes

• Expanded README.md with requirements, quick start, architecture, contribution guidance, and licensing.
• Added an MIT source-code license and THIRD_PARTY_NOTICES.md.
• Added CONTRIBUTING.md, CODE_OF_CONDUCT.md, and SECURITY.md.
• Added structured bug and feature issue forms plus a pull-request template.
• Added a macOS GitHub Actions build workflow for dev and master.
• Removed redundant Task hops from AppKit event callbacks and isolated window controllers to the main actor.

Testing

• Clean unsigned local Xcode Debug build succeeded.
• Both GitHub Actions macOS 14 builds passed.
• GitHub workflow and issue-form YAML parsed successfully.
• Character manifests and Package.resolved validated as JSON.
• Local README links resolve.
• Current-tree secret-pattern scan returned no findings.

Risks / follow-ups

• Historical media keeps the Git pack large; history cleanup would require a separate force-push decision.
• Tux and Surge asset terms should receive a final manual license review before public announcement.