Update to correct spelling of Argo CD

[terrytangyuan]

Summary

Follow up to address #569 (comment).

Checklist

• Tests pass locally (make test with -race)
• Linter passes (make lint)
• I did not skip/disable tests to make CI green
• I added/updated tests for new functionality
• I updated docs if user-facing behavior changed
• Changes follow existing patterns in the codebase
• Commits are cryptographically signed (git commit -S) — GPG signing info

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[yuanchen8911]

This PR updates most of the public docs terminology, but README.md still has three user-facing ArgoCD references at lines 7, 56, and 89, and site/docs/project/changelog.md still has two published ArgoCD references at lines 40 and 75. If those docs are intended to follow the same terminology cleanup, those references should be updated to Argo CD as well.

[yuanchen8911]

#570 (comment)

[terrytangyuan]

README.md still has three user-facing ArgoCD references at lines 7, 56, and 89

Are you looking at a different fork/branch? I don't see any "ArgoCD" in README.

site/docs/project/changelog.md still has two published ArgoCD references at lines 40 and 75.

I left that unchanged intentionally as you suggested in your original comment #569 (comment).