Improve English prose in high-traffic handbook guides

Background

The handbook now has English versions for the main content. Some high-traffic guides still need one more editing pass to read like native technical writing.

Scope

Start with the entry points and core guides:

• README.md
• 05-ai-native-development/05-ai-native-development_en/ai-native-git-workflow_en.md
• 05-ai-native-development/05-ai-native-development_en/codex-claude-code-git-practices_en.md
• 05-ai-native-development/05-ai-native-development_en/ai-generated-code-review_en.md
• 03-team-collaboration/03-team-collaboration_en/team-git-workflow-guide_en.md
• 04-github-engineering/04-github-engineering_en/github-engineering-governance_en.md
• 06-troubleshooting/06-troubleshooting_en/git-troubleshooting-playbook_en.md
• 10-company-practices/10-company-practices_en/company-practices-decision-map_en.md

Done When

• The edited guides keep the same structure and meaning.
• Awkward literal translations are replaced with natural technical English.
• Internal links still pass python3 scripts/check-docs.py.
• git diff --check passes.