[Chore] add PR policy and templates

[learncold]

Summary

• add a repository-level PR guide in CONTRIBUTING.md
• add a pull request template and CODEOWNERS file
• add a GitHub Actions PR policy check for title/body requirements

Related Issue

• Closes Chore: define PR policy and branch protection #36

Area

• Engine
• Domain
• Application
• Docs
• Build
• Analysis
• Chore

Architecture Check

• I kept the dependency direction application -> domain -> engine.
• I did not add Qt UI code to src/domain.
• I did not add domain or application dependencies to src/engine.
• I used src/ as the include root.

Verification

• cmake --preset windows-debug
• cmake --build --preset build-debug
• Not run (reason below)
• Local verification focused on repository file changes only.
• Existing configure step is currently blocked by vcpkg network download failure and a Visual Studio instance mismatch.

Risks / Follow-up

• The current repository still has src/application, src/domain, and src/engine directories without tracked source files, while CMakeLists.txt references missing C++ files.
• Branch protection will enforce PR metadata checks, but code build validation should be added after the native source tree is committed and CI prerequisites are stable.