fix: downgrade upload-artifact to v3 for Gitea compatibility

Gitea Actions only supports the v3 artifact protocol; upload-artifact@v4
uses the @actions/artifact v2 API which Gitea (identifying as GHES) rejects
with GHESNotSupportedError. The agent run step itself succeeds; only the
artifact upload was failing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/run-agent.yml

@@ -111,9 +111,11 @@ jobs:
             --max-runtime="$MAX_RUNTIME_SECONDS" \
             --output-dir=agent_run_output
 
+      # Gitea Actions 는 아티팩트 v3 프로토콜만 지원한다. upload-artifact@v4 는
+      # @actions/artifact v2 API 를 써서 Gitea(GHES 로 식별됨)에서 거부되므로 v3 사용.
       - name: Upload run artifacts
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: agent-run-${{ github.run_number }}
           path: agent_run_output/