Fix PR duplication detection workflow

[aaronpowell]

Summary

The PR duplication detection workflow is failing and not reliably flagging duplicate submissions.

Workflow links

• Workflow definition: https://github.com/github/awesome-copilot/blob/staged/.github/workflows/pr-duplicate-check.md
• GitHub run list: https://github.com/github/awesome-copilot/actions/workflows/pr-duplicate-check.lock.yml

Scope

• Reproduce and diagnose the current failure mode
• Implement a targeted fix to restore detection behavior
• Confirm expected behavior for duplicate and non-duplicate test scenarios
• Document any required config or secret expectations

Acceptance criteria

• Workflow completes successfully on staged
• Duplicate PRs are flagged with clear output
• Non-duplicate PRs are not falsely flagged
• Any required config changes are documented

Parent

Part of #2076