Refuse a backup destination an unplugged volume left empty (closes #239)

Every backup writes vaultik-destination.txt at the destination root.
While the local index holds snapshots, a destination without it is
not present: snapshot create refuses before writing, prune and
snapshot purge fail, and snapshot list and snapshot remove act as for
a destination that cannot be listed. snapshot create checks before it
drops incomplete snapshots, snapshot remove before it looks on the
destination. remote nuke deletes the marker last. The file:// backend
never creates its destination directory; that protects a directory on
a removable volume, not a mount point.

Tests back up into directories that exist; fixtures for a written
destination carry the marker.

Judgement call: the marker is rewritten on every backup.
Deviation: for the complexity limit, two blocks moved, unchanged, into
snapshotNamesToCreate and confirmRemoveSnapshot.

Model: opus-5-5

README.md

@@ -24,6 +24,10 @@ grep 'public key' vaultik_backup_private_key.txt
 vaultik config set age_recipients.0 age1YOUR_PUBLIC_KEY_HERE
 vaultik config set storage_url "file:///Volumes/usbstick/mybackup"
 
+# create the destination directory; vaultik never creates it, so a backup
+# to a volume that is not plugged in fails instead of filling the disk
+mkdir /Volumes/usbstick/mybackup
+
 # macOS only: grant your terminal app Full Disk Access first
 # (System Settings → Privacy & Security → Full Disk Access), otherwise
 # the backup will abort with a permission error on protected directories
@@ -298,6 +302,10 @@ value where a list belongs or a word where a number belongs), is refused.
   this duration instead of only the latest (e.g. `4w`, `30d`, `6mo`, `1y`).
   Without `--prune`, or with a value that is not a valid duration, the
   command fails before the backup starts.
+* Fails before it writes anything when a `file://` destination directory
+  does not exist, or when the local index holds snapshots and the
+  destination has no `vaultik-destination.txt` (see
+  [remote storage layout](#remote-storage-layout))
 
 **`snapshot list`**: Show every snapshot known to this host — the union
 of the local index and the backup destination store — with timestamps
@@ -327,7 +335,10 @@ invocation that reconciles them.
 
 If the destination store cannot be listed (unmounted volume,
 permission denied, network down), the command warns, falls back to the
-local index alone, and still exits zero.
+local index alone, and still exits zero. It does the same when the
+local index holds snapshots and the destination has no
+`vaultik-destination.txt`, as when a volume mounted at the destination
+is unplugged and leaves its empty mount point behind.
 * `--json`: Output in JSON format. Each entry carries `locally_tracked`
   (whether the snapshot is in the local index), `remote_key` (the full
   64-character storage key), and `remote_present` (whether it was seen
@@ -367,13 +378,14 @@ snapshot in the local index or on the destination store is an error;
 with `--local-only`, the snapshot must be in the local index. Local row
 cleanup (files, chunks, blobs the snapshot was the last referrer for)
 runs automatically. If removing the snapshot's metadata from the
-destination store fails (for example because the store is unreachable),
-the local-DB removal still completes and a warning is emitted; run
-`vaultik snapshot remove <snapshot-id>` again once the store is reachable
-to remove the snapshot's metadata from it (`vaultik prune` does not). For
-a snapshot that is only on the destination store, that failure is an
-error. To wipe everything on the destination in one go, use `vaultik
-remote nuke --force`.
+destination store fails (for example because the store is unreachable,
+or the local index holds snapshots and the destination has no
+`vaultik-destination.txt`), the local-DB removal still completes and a
+warning is emitted; run `vaultik snapshot remove <snapshot-id>` again
+once the store is reachable to remove the snapshot's metadata from it
+(`vaultik prune` does not). For a snapshot that is only on the
+destination store, that failure is an error. To wipe everything on the
+destination in one go, use `vaultik remote nuke --force`.
 * `--local-only`: Skip remote cleanup; only touch the local index
 * `--dry-run`: Show what would be deleted without deleting
 * `--force`: Skip confirmation prompt
@@ -404,8 +416,12 @@ manifest on the destination store to compute the still-referenced blob
 set and delete any blob not in that set. Step (3) reads all remote
 manifests — network cost scales with the number of snapshots. `snapshot
 create --prune` runs the same cleanup automatically; this is the
-manual entry point for the same work. If a blob cannot be deleted, the
-command (and `snapshot create --prune`) exits non-zero.
+manual entry point for the same work. If the destination store cannot
+be listed, or the local index holds snapshots and the destination has
+no `vaultik-destination.txt` (an unplugged volume's empty mount point),
+prune fails and deletes nothing. `snapshot purge` does the same. If a
+blob cannot be deleted, the command (and `snapshot create --prune`)
+exits non-zero.
 * `--force`: Skip confirmation prompt
 * `--json`: Output stats as JSON. Also skips the confirmation prompt, as
   `--force` does. A blob that cannot be deleted is counted in
@@ -429,11 +445,12 @@ since `prune` never deletes it.
 * `--json`: Output as JSON
 
 **`remote nuke`**: Delete every object under `metadata/` and `blobs/` on
-the backup destination store, whatever its name, leaving the bucket prefix
-empty. Destructive and irreversible. This is the single supported way to
-wipe the entire destination store. If a listing afterwards still finds
-anything there, the command names each object left and exits non-zero;
-running it again deletes what is left.
+the backup destination store, whatever its name, and then
+`vaultik-destination.txt`, leaving the bucket prefix empty. Destructive
+and irreversible. This is the single supported way to wipe the entire
+destination store. If a listing afterwards still finds anything under
+`metadata/` or `blobs/`, the command names each object left and exits
+non-zero; running it again deletes what is left.
 * `--force`: Required to confirm destruction.
 
 **`database delete`**: Delete the local SQLite state database file
@@ -458,7 +475,12 @@ object store. Credentials are read from `s3.access_key_id` and
 a local or mounted filesystem. Useful for testing or backing up to a NAS. The
 path must be absolute; a relative one is refused when the config loads.
 Percent-escapes in the path are decoded, so `my%20stick` and `my stick` name
-the same directory.
+the same directory. The directory must exist before the first backup: vaultik
+never creates it. A directory on a removable volume is gone while the volume is
+not mounted, so a backup then fails instead of writing to the disk under the
+mount point. The mount point itself stays behind when the volume is unplugged:
+there only `vaultik-destination.txt` stops such a backup, and only while the
+local index holds snapshots.
 
 **Rclone** (`rclone://remote/path`): Uses rclone's 70+ supported cloud
 providers. Requires rclone to be configured separately (`rclone config`).
@@ -489,6 +511,7 @@ For an implementation-level view of the internals — the data model, the
 
 ```
 <bucket>/<prefix>/
+├── vaultik-destination.txt     # Marker written by every backup
 ├── blobs/
 │   └── <aa>/<bb>/<full_blob_hash>
 └── metadata/
@@ -497,6 +520,15 @@ For an implementation-level view of the internals — the data model, the
         └── manifest.json.zst   # Unencrypted blob list (for pruning)
 ```
 
+* `vaultik-destination.txt` holds one fixed line of text, with no host
+  name, path or time. While the local index holds snapshots, a
+  destination without it is treated as not present: `snapshot create`
+  refuses to write to it, `prune` and `snapshot purge` fail, and
+  `snapshot list` and `snapshot remove` behave as for a destination that
+  cannot be listed. This keeps an unplugged volume's empty mount point
+  from being taken for a destination whose snapshots are all gone. Mount
+  the volume if it is removable; if the destination was emptied on
+  purpose, run `vaultik database delete` and back up again.
 * Blobs are two-level directory sharded using the first 4 hex chars of the blob hash
 * `db.zst.age` is a binary SQLite database (zstd compressed, age encrypted)
   containing all file metadata, chunk mappings, and relationships for the snapshot