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
+- Permission to write objects at the destination, and more on some rclone
-  [Destination permissions](#destination-permissions).
+  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
+- `snapshot create` without `--prune` writes objects. On S3 it needs no other
-  machine you back up can therefore hold credentials that can neither read nor
+  permission, so a machine you back up can hold credentials that can neither
-  delete.
+  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
+A backup tries to look up the marker file and each blob before it writes. On S3
-goes on when the destination refuses those look-ups.
+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/)

docs/DATAMODEL.md

@@ -11,8 +11,8 @@ The local index has two jobs:
 - It is the source of each snapshot's own database. A backup uploads a trimmed,
   encrypted copy of the index with every snapshot.
 
-A restore does not need the local index. It reads the copy that was uploaded
+A restore reads no file, chunk or blob rows from the local index. It reads the
-with the snapshot.
+copy that was uploaded with the snapshot.
 
 ## Terms
 

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.
+`snapshot create` without `--prune` writes objects. On S3 it needs no other
-Other commands need more:
+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:
 

internal/cli/remote.go

@@ -42,8 +42,8 @@ func newRemoteNukeCommand() *cobra.Command {
 		Short: "Delete ALL snapshot metadata and blobs from the backup destination store",
 		Long: `Removes every snapshot's metadata, every blob and the destination
 marker from the backup destination store, and each removed snapshot from
-the local index. After this command completes successfully the bucket
+the local index. After this command completes successfully the backup
-prefix is empty and the next backup starts from scratch.
+destination store is empty and the next backup starts from scratch.
 
 This is destructive and irreversible. Requires --force, and permission
 to list and delete objects on the destination store.`,

internal/cli/snapshot.go

@@ -69,7 +69,8 @@ If no names are provided, all configured snapshots are created.
 Needs at least one public key in age_recipients, and permission to write
 objects at the backup destination store. It does not need the age private
 key. With --prune it also needs permission to list, read and delete objects
-there.
+there. An rclone remote that can move objects also needs permission to
+read and delete.
 
 The config is read from the path given by --config or VAULTIK_CONFIG;
 otherwise from the platform config directory (~/.config/vaultik/config.yml