ci: upload-artifact v3 — v4 cannot run on this forge

Every paper compiled. The run's own check printed `missing: 0`, and the single
failing step is the artifact upload:

    ❌  Failure - Main actions/upload-artifact@v4
    @actions/artifact v2.0.0+, upload-artifact@v4+ and download-artifact@v4+
    are not currently supported on GHES

git.hanzo.ai is Gitea, which presents itself as GHES, and the v4 action refuses
on that basis before uploading anything. So the red badge was reporting an
action this forge cannot execute, not a broken paper — the worst kind of CI
failure, because it looks like the thing the job exists to check.

v3 speaks the artifact protocol the forge implements. Nothing else changes:
same name, same `pdfs/**/*.pdf` path, same 90-day retention, still
`if: always()` so the PDFs are captured even when a paper fails to build.

Co-authored-by: Hanzo Dev <[email protected]>

.hanzo/workflows/ci.yml

@@ -82,7 +82,13 @@ jobs:
           echo "missing: ${missing}"
           [ "$missing" -eq 0 ]
 
-      - uses: actions/upload-artifact@v4
+      # v3, NOT v4. This forge is Gitea, which presents as GHES, and the v4
+      # action refuses outright:
+      #   @actions/artifact v2.0.0+, upload-artifact@v4+ and download-artifact@v4+
+      #   are not currently supported on GHES
+      # Every paper compiled ("missing: 0") — this step alone failed the run, so
+      # the red badge was reporting an unusable action, not a broken paper.
+      - uses: actions/upload-artifact@v3
         if: always()
         with:
           name: papers