Prefer upstream repo when creating fork PRs

[felixfong227]

Summary

• Prefer the upstream repository as the PR base when both origin and upstream remotes are available.
• Keep fork PR detection aligned with the repository that will actually receive the PR, instead of always comparing against origin.
• Add a regression test covering a fork branch that should open a PR against the upstream repo with the correct owner-qualified head reference.

Testing

• Added a GitManager test for creating a fork PR against an upstream base repo.
• Not run: bun fmt
• Not run: bun lint
• Not run: bun typecheck

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Note

Medium Risk
Adjusts cross-repo PR detection to compare against upstream (when present) instead of always origin, which can change which head selectors are probed and where PRs are created. Risk is moderate because it affects PR creation/lookup logic across different remote configurations.

Overview
When both origin and upstream remotes are configured, PR creation/lookup now treats upstream as the target/base repository (falling back to origin when upstream is missing) when determining whether a branch is cross-repo.

Adds a regression test that simulates a forked origin branch opening a PR against an upstream repo, asserting gh pr create uses the owner-qualified head selector (e.g. owner:branch) and the upstream base branch.

^{Reviewed by Cursor Bugbot for commit 1b72627. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Prefer upstream remote over origin when resolving base repository for fork PRs

When creating a PR, makeGitManager.resolvePullRequest now selects the upstream remote as the target base repository when it is configured, falling back to origin otherwise. isCrossRepository is computed by comparing the head remote against this resolved target rather than always against origin. A new test in GitManager.test.ts covers the case where both origin and upstream remotes exist and verifies the PR is created with the fork head against the upstream base.

^{Macroscope summarized 1b72627.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR changes runtime behavior in how PRs are targeted when working with forks - PRs will now be created against the upstream repository instead of origin. While the change is small and well-tested, it modifies core PR creation logic and warrants human review.

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