[codex] Fix main CI generator dependency pin

[justin808]

Summary

• format the agent workflow README so main's Prettier check passes
• pin the generator e2e React template to @babel/preset-react@^7.0.0
• add a guard spec so the template stays Node 20-compatible

Root cause

Main CI started resolving unpinned @babel/preset-react to a package path that pulls Babel 8-era helpers requiring newer Node than the generator workflow's Node 20.19.x runtime. The React install then failed, and the generated app later failed to compile because react/react-dom were missing.

Validation

• yarn prettier --check .
• .agents/bin/lint
• .agents/bin/build
• bundle exec rspec spec/generator_specs/generator_spec.rb:15
• Node 20 CI-style generator repro for the failing npm and yarn_classic examples with SHAKAPACKER_NPM_PACKAGE set
• git diff --check
• autoreview clean

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[justin808]

Closing as superseded by the merged fixes: #1180 fixed the agent README formatting that was part of this draft, and #1167 merged the generator e2e dependency pin with a stronger React 18 + preset-react 7 constraint and full CI coverage. No separate issue work is included in this closeout wave.