fix(agent): avoid redundant PR checkout

Make existing-PR follow-up context check out the branch only when needed, preventing cloud agents from repeating the checkout on every prompt.

Generated-By: PostHog Code
Task-Id: 9e493924-1eab-496a-ba66-aba8366de96e

Author: tatoalo

packages/agent/src/server/agent-server.test.ts

@@ -3827,13 +3827,18 @@ describe("AgentServer HTTP Mode", () => {
       expect(context).not.toContain("gh pr checkout");
     });
 
-    it("returns auto-update PR context for Slack-origin runs", () => {
+    it("avoids redundant PR checkout for auto-update runs", () => {
       process.env.POSTHOG_CODE_INTERACTION_ORIGIN = "slack";
       const s = createServer();
       const context = (s as unknown as TestableServer).buildDetectedPrContext(
         prUrl,
       );
-      expect(context).toContain(`gh pr checkout ${prUrl}`);
+      expect(context).toContain(
+        `If it is not already checked out, check it out with \`gh pr checkout ${prUrl}\``,
+      );
+      expect(context).toContain(
+        "Do not check it out again when it is already active",
+      );
       expect(context).toContain(
         "Make changes, commit, and push to that branch",
       );

packages/agent/src/server/agent-server.ts

@@ -3152,7 +3152,7 @@ export class AgentServer {
       `IMPORTANT — OVERRIDE PREVIOUS INSTRUCTIONS ABOUT CREATING BRANCHES/PRs.\n` +
       `You already have an open pull request: ${prUrl}\n` +
       `You MUST:\n` +
-      `1. Check out the existing PR branch with \`gh pr checkout ${prUrl}\`\n` +
+      `1. Continue working on the existing PR branch. If it is not already checked out, check it out with \`gh pr checkout ${prUrl}\`. Do not check it out again when it is already active.\n` +
       `2. Make changes, commit, and push to that branch\n` +
       `You MUST NOT create a new branch, close the existing PR, or create a new PR.`
     );