docs: add archive notice to README

[alkalescent]

Summary

• Adds a visible warning banner to the top of the README indicating that the repository has been archived and is no longer actively maintained.

Test plan

• Verify the warning renders correctly on GitHub

Summary by CodeRabbit

• Documentation
  • Added an archived repository warning to the README to inform users that the repository is no longer actively maintained.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: f2c867c0-eaf5-44d2-ad9d-38da4dbb3fa9

📥 Commits

Reviewing files that changed from the base of the PR and between 7126cea and 1172608.

📒 Files selected for processing (1)

• README.md

📝 Walkthrough

Walkthrough

A warning callout is added to the README introduction indicating the repository has been archived and is no longer actively maintained, with no changes to other documentation content or functionality.

Changes

Cohort / File(s)	Summary
Documentation Update README.md	Adds a prominent warning callout to the introduction indicating the repository is archived and no longer actively maintained.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🐰 A repository's final bow,
"Archived," now whispered in the README's glow,
No updates, no fuss, just a notice so bright,
Gentle reminder to those passing by night! 📖✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'docs: add archive notice to README' is directly related to the changeset, which adds an archive notice warning to the README documentation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/archive-notice

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[github-actions]

X-Test Failure Report

[gemini-code-assist]

Code Review

This pull request updates the README.md file to include a warning notice indicating that the repository has been archived and is no longer actively maintained. There are no review comments to address, and I have no feedback to provide.

[alkalescent]

For context, source code and CI checks have already been migrated to opentdf/platform.

[github-actions]

X-Test Results

✅ go-main
✅ go-v0.9.0
✅ java-main
✅ java-v0.9.0
✅ js-main
✅ js-v0.9.0
test-cases-mapping-report
bats-test-results