fix(repo): sync a branch without switching to it

tea repo sync ran git switch on the synced branch, so syncing main while
on experiment left the user on main. A branch that is not checked out now
moves with update-ref, guarded by its old value; the checked-out branch
merges with --ff-only and needs a clean working tree; a missing branch is
created and set to track the remote. Diverged branches stay untouched.

Sync reports "✓ Synced main from origin/main (fast-forward a..b)" on a
terminal, "already up to date" when nothing changed, and the JSON result
gains the previous commit. Outside a clone, or without a remote for the
repository, the errors now name repo sync instead of checkout.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/app_test.go

@@ -514,7 +514,6 @@ const (
 	repo list
 	repo migrate
 	repo set-default
-	repo sync
 	repo view
 	search issue
 	search pr