Chore: define PR policy and branch protection

[learncold]

Summary

• Add repository PR rules, template, and branch protection for main.

Why

• The repository currently has no protected branch, no PR template, and no automated PR policy check.
• This makes it easy to bypass the layered architecture rules and merge incomplete PR descriptions.

Scope

• Add repository-side PR guidance files.
• Apply GitHub branch protection for main.
• Keep the setup lightweight enough for the current single-maintainer stage.