fix(contributor-check): raise artifact retention to 90 days (#4618)

The contributor-check-result artifact was uploaded with retention-days: 3.
eng/submission-gate.mjs's readContributorRiskArtifact() downloads that
artifact via 'gh run download' to read the contributor risk signal. Once
the artifact expires, the download fails, is caught and treated as null,
and the gate reports a permanent 'Contributor risk signal' infrastructure
failure for any PR that takes longer than 3 days to review -- only
clearable via a maintainer commenting /rerun-checks.

Reproduced: 'gh run download 37007671556 --repo github/awesome-copilot
--name contributor-check-result' returned 'no valid artifacts found to
download' because the artifact (uploaded 2026-10-02, 338 bytes, upload
itself succeeded) expired under the 3-day retention before PR #3871 was
reviewed.

Raise retention-days to 90 (GitHub's standard default) so the artifact
survives regardless of review latency.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 211fb092-66c2-4049-b1b9-614067c2eefe

.github/workflows/contributor-check.yml

@@ -420,7 +420,7 @@ jobs:
           name: contributor-check-result
           path: ${{ runner.temp }}/contributor-check-result/result.json
           if-no-files-found: error
-          retention-days: 3
+          retention-days: 90
 
       - name: Job summary
         if: always()