Add CODEOWNERS for review routing

[nojibe]

Problem

There is no .github/CODEOWNERS. As outside contributions grow, there's no automatic reviewer assignment and no clear ownership of sensitive areas (CLI eval engine, blueprint format, scoring/points logic, infra config).

Proposed change

Add .github/CODEOWNERS mapping default ownership plus targeted owners for high-risk directories, e.g.:

• * → core maintainer(s)
• /src/cli/ → eval-engine owners
• /docs/BLUEPRINT_FORMAT.md, blueprint parsing code → format owners
• /.github/, workflow/infra files → maintainers

Acceptance criteria

• .github/CODEOWNERS exists with a valid default owner
• Critical paths have explicit owners
• Branch protection (if enabled) requires code-owner review on those paths