rsync: recursive mirroring between local and remote

[lionello]

There's no way to mirror a tree. cp -R exists but is one-shot and one-directional (remote → local only), re-downloads everything on every run, and has no delete/prune.

Proposed: onedrive rsync [-n] [-r] [--delete] SOURCE DEST, using the same ./ and :/ prefixes as cp/mv.

Building blocks:

• SHA1 comparison to skip unchanged files — cp: skip the transfer when the SHA1 already matches #36
• recursive enumeration — cp_paged() / findItems()
• GET /drive/root/view.delta for cheap "what changed remotely", with the delta token persisted between runs
• fileSystemInfo set at upload time so local mtimes survive the round trip

The last two are internal to this command. #43 and #52 are closed as not planned because the user-facing delta and touch verbs aren't wanted — that doesn't block using the same endpoints as plumbing here.

Open questions worth settling before implementing:

• one-way (local → remote, remote → local) only, or bidirectional?
• --delete semantics, given remote deletes are recursive and land in the recycle bin
• where the delta token lives — alongside the auth token in $XDG_STATE_HOME?

Starting one-way with -n/--dry-run and no --delete would already cover most of the value.