ghstack clean: remove orphan branches

[vmoens]

Adds ghstack clean to delete orphan ghstack branches from the remote.

Over time, gh/{user}/{num}/* branches pile up on the remote with no associated open PR. This happens more often than you'd think:

• Dropping a commit during git rebase -i after the PR was created
• Closing/merging a PR manually from GitHub UI instead of using ghstack land
• Squashing commits that had separate PRs
• ghstack land or ghstack submit failing halfway through

These branches just sit there forever and there was no way to clean them up. Now there is.

Tested "in prod" in tensordict and torchrl with success