docs: condense contributing review notes

[mfoltz]

Motivation

• Keep the release-process rules and branch/channel policy intact while removing reviewer-only troubleshooting details to keep contributor-facing docs concise and focused.

Description

• Edit .github/CONTRIBUTING.md to preserve the ## Release process section unchanged and replace the ## Review/testing note with two short bullets: run bash .codex/install.sh before local checks and do not run bash -n on .github/workflows/*.yml, removing the inline Python/PyYAML examples and the ModuleNotFoundError troubleshooting guidance.

Testing

• Ran git diff --check, which completed with no issues.

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Codex Task

[Copilot]

Pull request overview

This PR condenses contributor-facing guidance by trimming reviewer-specific troubleshooting details from the contributing docs while keeping the release policy documentation intact.

Changes:

• Preserved the existing ## Release process section unchanged.
• Replaced the detailed ## Review/testing note troubleshooting content with two concise bullets focused on environment setup and correct validation scope.