Ability to default PRs to an upstream

### Describe the feature or problem you'd like to solve

Many enterprises or Open Source require you to fork the repo and make PRs from your fork. Instead of defaulting PRs to the fork, I'd like an option to open them in the upstream on my behalf

### Proposed solution

Whatever repo is set with 'gh repo set-default` (assuming the user also uses GH CLI) can be referenced for other open issues or pull requests. Being able to set the default repo for pull requests still allows the agent to work in the worktree of the fork but contribute to the end repo the user will eventually need to PR the code into

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by tracing how pull-request targets are selected and how the CLI reads the repository set by `gh repo set-default`. Done means a user working from a fork can create a pull request targeting that configured upstream while the agent continues using the fork's worktree.

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