[codex] Add GitHub CLI to dev bootstrap

[codeforester]

Summary

Adds GitHub CLI (gh) to Base's opt-in developer bootstrap path.

What Changed

• Adds gh as a Homebrew-managed developer dependency when basectl setup --dev is used.
• Includes gh in basectl check --dev, including JSON check output.
• Keeps normal basectl setup and basectl check unchanged: GitHub CLI remains opt-in behind --dev.
• Updates setup help text and README wording from BATS-only developer dependencies to BATS plus GitHub CLI.
• Extends the BATS Homebrew stubs and assertions for install, dry-run, check, and JSON check behavior.

Why

GitHub's official command-line tool is gh, and Base's developer setup already has an opt-in --dev lane for tools useful while working across repositories. Adding gh there makes GitHub workflows available without making it part of the minimal bootstrap.

Impact

Developers who run basectl setup --dev will get both BATS and GitHub CLI installed through Homebrew. Developers who run regular setup will not install or check for gh.

Validation

• bats cli/bash/commands/basectl/tests/setup.bats passed all 40 tests.