chore: make crates.io publishing manual

[smiggleworth]

Summary

Make crates.io publishing manual-only and require the workflow to be dispatched from a release tag.

Why are you making this contribution?

Publishing should be an intentional release action rather than an automatic response to every tag push.

Linked issues

No issue: release workflow policy.

Acceptance audit

• Criterion: Publishing is manual and cannot run from a non-tag ref.
  Evidence: The workflow uses only workflow_dispatch and checks GITHUB_REF_TYPE=tag before the release gate.
  Production entry point: .github/workflows/publish.yml publishes cntryl-midge through the existing release gate and OIDC job.
  Resolution: Matches the requested manual publishing approach.

Risk and compatibility

Existing release validation, required checks, and OIDC authentication remain unchanged.

Verification

• Workflow structure reviewed.
• git diff --check passed.

Tool assistance disclosure

• AI or another generative tool materially assisted this contribution.
  The workflow change was reviewed and validated before submission.

Contributor responsibility

• I understand the complete change and can explain or revise it.
• I reviewed the complete diff.
• I reported validation accurately and did not claim checks I did not run.
• I disclosed material generated assistance.
• I have the right to submit this work under the repository's license.

Review checklist

• Behavioral tests use the real production entry point, not only private helpers.
• New enum variants update tests/coverage_manifests.rs.
• Coexisting implementations have a differential test, or the unused variant is removed.
• Shared test infrastructure uses poison-tolerant locks and consistent failpoint gates.
• Failpoint governance relies on mechanical call-site discovery, not a hardcoded allowlist.
• Expensive coverage and mutation checks remain scheduled/manual unless their CI cost is explicitly approved; Sqrzl remains the self-contained cloud qualification authority.
• A real-provider behavior difference is reproduced in Sqrzl and retained as a Midge regression test where applicable.
• Every linked issue criterion is represented above; this workflow-only change has no linked issue criteria.

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