ci(workflows): downgrade upload-artifact v4 → v3 — Gitea act_runner limitation (ADR-014)

Reverts the re-regression introduced in 410b91e2. Gitea Actions
(act_runner) does not implement the v4 artifact protocol — jobs report
failure even when all tests pass. Pins all three call sites back to @v3
and adds load-bearing inline comments pointing to ADR-014 / #557.

This commit makes the grep guard added in the previous commit GREEN.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/coverage-flake-probe.yml

@@ -56,9 +56,10 @@ jobs:
             exit 1
           fi
 
+      # Gitea Actions (act_runner) does not implement upload-artifact v4 protocol — pinned per ADR-014. Do NOT upgrade. See #557.
       - name: Upload coverage log on failure
         if: failure()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: coverage-log-run-${{ matrix.run }}
           path: /tmp/coverage-test-${{ github.run_id }}-${{ matrix.run }}.log