Say what a backup needs on an rclone remote and fix four more findings

- `README.md`, `docs/REPOSTRUCTURE.md`, `internal/cli/snapshot.go`: a
  backup needs only write permission on S3; an rclone remote that can
  move objects also needs read and delete, and an upload there stops
  when its look-up is refused.
- `docs/DATAMODEL.md`, `docs/REPOSTRUCTURE.md`: a restore reads no
  file, chunk or blob rows from the local index, but it does open it.
- `README.md`: confirmation prompts go to stdout.
- `internal/cli/remote.go`: `remote nuke` leaves the backup destination
  store empty, not a bucket prefix.
- `README.md`: a TODO section, as `REPO_POLICIES.md` requires.

Model: fable-5-1

docs/REPOSTRUCTURE.md

@@ -105,7 +105,7 @@ This has two consequences:
 A backup looks a blob up by name before it uploads it. It does not upload a
 blob that is already there.
 
-When the destination refuses the look-up, the backup uploads the blob. An
+On S3, when the destination refuses the look-up, the backup uploads the blob. An
 object with that name is then replaced by one that holds the same chunk data.
 
 ## Metadata
@@ -314,8 +314,8 @@ Trust the temporary directory as much as the directory you restore into.
 
 ### Commands need different permissions
 
-`snapshot create` without `--prune` needs only permission to write objects.
-Other commands need more:
+`snapshot create` without `--prune` writes objects. On S3 it needs no other
+permission. Other commands need more:
 
 - These commands list and read:
     - `snapshot list`
@@ -328,6 +328,10 @@ Other commands need more:
     - `remote nuke`
 - `prune` lists, reads and deletes.
 
+An rclone remote that can move objects also needs permission to read and
+delete. An upload there looks up the object it replaces and deletes it. It stops
+when the look-up is refused.
+
 `snapshot verify` and `snapshot restore` list only when they are given a remote
 key. `snapshot remove --local-only` does not reach the destination.
 `snapshot remove --dry-run` deletes nothing.
@@ -421,7 +425,8 @@ To avoid this case, give each machine and each user its own destination.
 - the snapshot's `db.zst.age`
 - the blobs that hold the files to restore
 
-A restore does not read the manifest and does not need the local index.
+A restore does not read the manifest, or the file, chunk and blob rows of the
+local index.
 
 It works in these steps: