Adds Number alongside every task id in External/'s DTOs -- the two
central mappers (ToDto/ToRefDto -> TaskDto/TaskRefDto) plus every
DTO that carries a bare task id and bypasses them (batch results,
queue state, wait-for-change, config, attachments, handoff,
lifecycle, merge-preview-set, worktree list). Input resolution
(#123 as an argument) stays for slice 3.