fix: Woodpecker reports error for evidence-only PRs — no matching pipeline triggers (#1153)

Add a lightweight always-run passthrough pipeline that triggers on all PRs
and exits 0, ensuring every PR gets at least one successful CI status.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.woodpecker/passthrough.yaml

@@ -0,0 +1,12 @@
+kind: pipeline
+type: docker
+name: passthrough
+
+when:
+  - event: pull_request
+
+steps:
+  - name: pass
+    image: alpine
+    commands:
+      - echo "ok"