fix(readme): correct Codex marketplace install command

[kantorcodes]

Summary:

• replace raw marketplace JSON URL with Git repo marketplace command Codex expects
• include sparse paths for marketplace manifest and mirrored plugin bundles
• document why raw GitHub file URLs fail with remote 404

Closes #64.

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 files)

• README.md - Documentation-only changes fixing marketplace command instructions

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_{Reviewed by laguna-m.1-20260312:free · 167,866 tokens}

[gemini-code-assist]

Code Review

This pull request updates the README.md to use a Git repository URL instead of a raw file URL for the codex plugin marketplace add command, incorporating flags for sparse checkouts and adding a warning about raw URL failures. A suggestion was made to wrap the repository URL in double quotes for better shell compatibility.