docs: document test pull request validation

[gregkonush]

Summary

• Adds a README note that docs-only pull requests can validate repository PR automation.
• Keeps the change isolated to documentation with no runtime behavior changes.
• Uses this branch as a ready-to-merge test pull request.

Related Issues

None

Testing

• bunx oxfmt --check README.md - passed

Screenshots (if applicable)

N/A

Breaking Changes

None

Checklist

• Testing section documents the exact validation performed (or N/A with justification).
• Screenshots and Breaking Changes sections are handled appropriately (removed or filled in).
• Documentation, release notes, and follow-ups are updated or tracked.

[gregkonush]

Status: PR ready to merge.

• Implemented README-only test PR note.
• Local validation passed: bunx oxfmt --check README.md.
• CI visible for head 55f560a93458a7bda157458b4297583ac79949d4 passed: Semantic Commits, Semantic Pull Request.
• PR: docs: document test pull request validation #6575

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