## Summary
- Add workflow step to configure `upstream` remote pointing to base repo
for cross-repo PRs
- Fetch `upstream/main` before Claude runs so rebase target is available
- Update pr-rebase skill to explicitly use `upstream/main` for
cross-repo PRs (falling back to `origin/main` for same-repo PRs)

The claude-rebase workflow was failing because only `origin` (pointing
to the fork) was configured. Claude would rebase onto `origin/main`
which may be stale instead of the base repo's current main.

## Test plan
- Trigger the cc:rebase label on a cross-repo PR and verify it rebases
onto the correct upstream/main

#skip-bugbot

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fixes cross-repo PR rebasing by adding an upstream remote to the base
repo and rebasing onto upstream/main instead of a stale origin/main.
This makes the claude-rebase workflow reliable for forks.

- **Bug Fixes**
  - Add upstream remote in workflow and fetch upstream/main.
- Update pr-rebase command to prefer upstream/main, fallback to
origin/main for same-repo PRs.
  - Document cross-repo remote setup and permissions in AGENTS.md.

<sup>Written for commit 52ea2741fa0397ee9e783c9a19b0d18cc54e388c.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>