ci: run agent-smoke only on PRs, not push events

Push events test the raw branch which may be behind main.
PR events test the merge result, which is what matters.
This eliminates false CI failures that block the dev-agent.

.woodpecker/ci.yml

@@ -16,6 +16,8 @@ steps:
 
   - name: agent-smoke
     image: alpine:3
+    when:
+      event: pull_request
     commands:
       - apk add --no-cache bash
       - bash .woodpecker/agent-smoke.sh