Rebasing from a worktree fails if main is checked out

[rogueg]

While on a worktree, I got

> gh stack rebase
✓ Fetched origin
⚠ Failed to fast-forward main: failed to run git: fatal: cannot force update the branch 'main' used by worktree at '/co/main'

Stack detected: (main) <- tf/prod <- tf/subnets
Rebasing branches in order, starting from tf/prod-name to tf/subnets
✓ Skipping tf/prod (PR #99 merged)
⚠ Rebasing tf/subnets onto main — conflict

I was able to fix it by detaching from the main branch in the main worktree. There were no true conflicts in this stack.

Seems to me there are two issues:

1. stacks should run against origin/main, rather than local main.
2. failing to update main should abort immediately, rather than attempt the rebase.

[johanbrandhorst]

This is exacerbated by AI tools like Zed and Claude Desktop that manage worktrees for you - because gh stack rebase doesn't error in these cases, they can get confused and then refuse to use the tool altogether. The fix is the same as suggested by OP, but I agree with both points of feedback.