fix(trail): let a budget-limited branch scan pass the update guard

A branch with no trail (main) never matches, so the guard's scan reads every page; once a repository has more than 2,000 trails it stopped at the page budget and refused every bare-number update from such a branch. gh/entireio/cli is at about 1,740 and growing. Treat reaching the budget without a match as no trail, with a warning: the list is ordered by recent activity, so a branch being worked on is found early. Lookup errors, timeouts, detached HEAD, and missing parents still refuse.