[codex] structure guide challenge tips

[Sean-Kenneth-Doherty]

Summary

• replace inline dash-separated guidance in the guide page with a semantic nested list
• make the “Navigating Large Codebases” tips easier to scan
• preserve the existing Common Challenges section and copy intent

Addresses #501

Validation

• git diff --check
• source check confirms the inline process: - Start dash-run text is removed
• source check confirms the four large-codebase tips now render as list items
• GITHUB_TOKEN=$(gh auth token) pnpm build
• generated guide HTML keeps the Common Challenges section and includes the nested tip list

[Sean-Kenneth-Doherty]

Validation notes for this guide-page content cleanup:

• git diff --check passes
• source check confirms the inline process: - Start dash-run text is removed
• source check confirms the four large-codebase tips now render as list items
• GITHUB_TOKEN=$(gh auth token) pnpm build passes
• generated guide HTML keeps the Common Challenges section and includes the nested tip list