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

README.md

@@ -311,8 +311,8 @@ It needs:
 
 - At least one public key in `age_recipients`.
 - Read access to the configured paths.
-- Permission to write objects at the destination. See
-  [Destination permissions](#destination-permissions).
+- Permission to write objects at the destination, and more on some rclone
+  remotes. See [Destination permissions](#destination-permissions).
 - Free temporary space. See [Temporary files](#temporary-files).
 - The [lock](#locking).
 
@@ -1025,9 +1025,9 @@ A leftover `.partial` file is ignored. You can delete it.
 
 Commands need these permissions at the destination:
 
-- `snapshot create` without `--prune` needs only permission to write objects. A
-  machine you back up can therefore hold credentials that can neither read nor
-  delete.
+- `snapshot create` without `--prune` writes objects. On S3 it needs no other
+  permission, so a machine you back up can hold credentials that can neither
+  read nor delete.
 - These commands list and read:
     - `snapshot list`
     - `snapshot verify`
@@ -1039,6 +1039,10 @@ Commands need these permissions at the destination:
     - `remote nuke`
 - `prune` and `snapshot create --prune` list, read and delete.
 
+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 you give a remote key.
 `snapshot remove --local-only` does not reach the destination.
 `snapshot remove --dry-run` deletes nothing.
@@ -1046,8 +1050,8 @@ Commands need these permissions at the destination:
 A machine that runs `snapshot create --prune` can delete its own backups. To
 keep it to write-only credentials, run `prune` from a separate, trusted machine.
 
-A backup tries to look up the marker file and each blob before it writes. It
-goes on when the destination refuses those look-ups.
+A backup tries to look up the marker file and each blob before it writes. On S3
+it goes on when the destination refuses those look-ups.
 
 ### The marker file
 
@@ -1166,6 +1170,7 @@ progress:
 stdout carries the output you asked for:
 
 - status and progress lines
+- confirmation prompts
 - warnings, and the errors that `snapshot create` and `snapshot restore` print
   about files
 - tables and reports
@@ -1477,6 +1482,10 @@ Each archive holds three files:
 The release files are not signed, have no software bill of materials and are
 not built reproducibly.
 
+## TODO
+
+Planned work is listed in [`TODO.md`](TODO.md).
+
 ## license
 
 [MIT](https://opensource.org/license/mit/)