chore: add CODEOWNERS

[bh2smith]

What

Adds a .github/CODEOWNERS file assigning @bh2smith as the default owner for all paths.

Why

Brings ownership parity with the other Dune API client SDKs and ensures reviewers are auto-requested on PRs. Mirrors the same change being made across the TS and Rust client repos.

Adjust the owner(s) if a team handle (e.g. a @duneanalytics/... team) is preferred.

[cursor]

PR Summary

Low Risk
Only adds GitHub metadata for review routing; no application, security, or runtime code changes.

Overview
Adds a new .github/CODEOWNERS file so GitHub auto-requests reviewers on PRs that touch matched paths.

The rule @bh2smith owns *, so every path in the repo falls under that default ownership—aligned with the other Dune API client SDKs.

^{Reviewed by Cursor Bugbot for commit 4bc31e3. Configure here.}