Fix `partio rewind` when checkpoint branch isn't fetched locally

[jcleira]

partio rewind --list returns "no checkpoint branch found" when partio/checkpoints/v1 doesn't exist locally. If checkpoints were pushed from another machine or a clone, the branch may exist on the remote but not be fetched. The current error is confusing because it implies checkpoints don't exist at all.

Fix

In runRewindList() (cmd/partio/rewind.go), when local branch verification fails:

1. Check if the branch exists on the remote: git ls-remote --heads origin partio/checkpoints/v1
2. If found on remote, print a helpful message:

   Checkpoint branch not fetched locally.
   Run: git fetch origin partio/checkpoints/v1:partio/checkpoints/v1
   

3. If neither local nor remote has the branch, keep the current error.

Apply the same check in runRewindTo() for consistency.

Program

See: .minions/programs/fix-rewind-checkpoint-not-fetched.md

Inspired by: entireio/cli changelog v0.5.3 — "Resume failing when checkpoints aren't fetched locally yet"