feat: support fork PRs in rebase-pr action (#11)

[tomgrv]

What

Updated rebase-pr/run.sh to detect cross-repository (fork) PRs and dynamically configure a separate Git remote for the head repository, enabling fetch and push to/from the fork instead of always assuming origin.

Why

Closes #11. Previously the script assumed both branches were on origin, causing it to fail for PRs submitted from forks — a common open-source workflow.

Originally flagged by @gemini-code-assist in #10.

How

Uses gh pr view --json isCrossRepository,headRepository to detect fork PRs. If cross-repo:

• Adds a temporary fork-remote pointing to the fork's clone URL
• Fetches the head branch from fork-remote instead of origin
• Pushes the rebased branch back to fork-remote
• Cleans up the temporary remote via trap on exit

Same-repo PRs continue using origin with no behavioural change.

Checklist

• Code follows project conventions
• Tests added or updated
• Documentation updated (rebase-pr/README.md — mention fork PR support)
• No breaking changes
• Ready for review

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the rebase-pr action by adding support for pull requests submitted from forks. By dynamically detecting the source repository and configuring temporary remotes, the action now correctly handles rebasing and pushing changes for cross-repository contributions, ensuring a seamless workflow for open-source maintainers.

Highlights

• Fork PR Detection: Integrated gh pr view with JSON output to identify cross-repository pull requests.
• Dynamic Remote Configuration: Added logic to dynamically create and clean up a temporary Git remote for fork-based head repositories.
• Rebase Workflow: Updated fetch and push operations to target the correct remote based on whether the PR originates from a fork or the main repository.

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[gemini-code-assist]

Code Review

This pull request introduces a shell script for automated PR rebasing that leverages the GitHub CLI and temporary git worktrees, including support for fork repositories. The review feedback highlights a critical bug where the script could crash on non-fork PRs due to null JSON property access and suggests refactoring duplicated logic for cleanup traps and output generation into reusable functions to improve maintainability and follow DRY principles.

[tomgrv]

/rebase