fix(workflow): check PR URL existence instead of branch for hasExistingPr

prepareImplementationBranch sets state.pullRequest with branch info
before the PR is actually created on GitHub. This caused the workflow
to always take the 'update PR' path instead of 'create PR' on the
first implementation pass. Now checks state.pullRequest?.url instead.

Author: zhangweijian97

src/core/workflow.ts

@@ -646,7 +646,7 @@ async function handleImplementingStage(
 		}
 	}
 
-	const hasExistingPr = Boolean(state.pullRequest);
+	const hasExistingPr = Boolean(state.pullRequest?.url);
 	const fixRound = hasExistingPr && state.bugs.length > 0;
 	const prompt = fixRound
 		? await buildFixPrompt(