Let cloud task users override the single-PR default

[tatoalo]

Problem

Cloud task runs with an existing open PR hard-blocked the agent from creating a new branch or PR ("You MUST NOT create a new branch, close the existing PR, or create a new PR"). Users who explicitly authorized additional PRs still got refused, since the instruction had no override.

Changes

Made the single-PR prohibition the default rather than absolute in the existing-PR cloud task prompts (packages/agent/src/server/agent-server.ts): continue on the existing PR unless the user explicitly asks for a new branch or separate PR. This matches the escape hatch already used in the non-auto-publish path, so the default (avoid spontaneous extra PRs) is preserved while explicit user authorization is honored.

How did you test this?

Text-only prompt-string change with no assertions against the edited lines. Typecheck and the surrounding test suite fail only on pre-existing environment limitations (unbuilt workspace deps and the sandbox's global git commit block used by test fixtures) — none related to this change.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog from a Slack thread

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit b665187.}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
packages/agent/src/server/agent-server.ts:3299-3302
**Override Conflicts With Mandatory Checkout**

When the user explicitly requests a separate branch or PR, the preceding `MUST` steps still require checking out the existing PR and pushing to that branch. The agent can therefore update the existing PR before applying the new exception, so the explicit request is misapplied.

```suggestion
      `Unless the user explicitly asks for a new branch or separate PR, you MUST:\n` +
      `1. ${this.buildExistingPrCheckoutInstruction(prUrl)}\n` +
      `2. Make changes, commit, and push to that branch\n` +
      `By default, do not create a new branch, close the existing PR, or create a new PR — continue on the existing PR. If the user explicitly asks for a new branch or separate PR, follow their instruction instead.`
```

_{Reviews (1): Last reviewed commit: "Let cloud task users override the single..." | Re-trigger Greptile}