Update CODEOWNERS for proto ownership

[BenHenning]

Explanation

This splits code ownership for model & script protos. We originally intended for all protos to go into model/ per Gradle limitations, but scripts/ is Bazel-only and is demonstrating our longer-term strategy of putting protos closer to code. Ownership is being updated accordingly.

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

For UI-specific PRs only

N/A

[BenHenning]

Thanks @seanlip!

[oppiabot]

Unassigning @seanlip since they have already approved the PR.

[oppiabot]

Hi @BenHenning, this PR is ready to be merged. Please address any remaining comments prior to merging, and feel free to merge this PR once the CI checks pass and you're happy with it. Thanks!