file provider #836

Epic: #815 — Providers. The file provider (pkg/op/provider/file): the four sealed variants, the writers
(link, copy, move, backup, mkdir, write_text, write_bytes), the readers, traversal, and their
compensations. Every file-provider task, fix, and chore files here.

Known members at filing: #822 (file.mkdir ignores --conflict). The file API features under the resource model
(#460) predate this scheme and stay where they are.