fix(readme): use literal quoting for marketplace URL

[kantorcodes]

Summary:

• use single quotes around Codex marketplace Git repo URL
• addresses Gemini review from PR 107 after PR auto-merged

Verification:

• README install command check passed
• marketplace JSON validates with jq
• git diff --check passed
• README guidance guard script passed

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (1 file)

• README.md - No issues

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_{Reviewed by step-3.5-flash · 69,153 tokens}

[gemini-code-assist]

Code Review

This pull request updates the README.md file to use single quotes for a URL in a shell command example. The reviewer suggests extending this literal quoting to all other arguments in the command to ensure consistency and prevent unintended shell expansion.