Keep searching past an unreadable baseline, and degrade when none can be read

A baseline copy that could not be read (permissions, symlink policy, a transient I/O error) ended the whole multi-worktree lookup, and the caller then had no baseline at all, so every untracked file counted as new: the over-capture the lookup exists to prevent. The search now skips that copy and tries the next worktree; when a baseline exists but none can be read, the loaders return it marked unreadable, which disables status-based new-file detection. The TodoWrite checkpoint hook applies the same guard.