Read the snapshot name using the stored hostname (closes #230)

A snapshot ID is hostname_name_timestamp, and purge took the name to be
everything between the first and the last underscore. With a hostname
such as my_host the name home came out as host_home, so
`snapshot purge --keep-latest --snapshot home` found nothing to delete
and `snapshot create --prune` purged nothing without a message. The name
is now read by removing the hostname stored with the snapshot, in the
short form the ID uses, so both may contain underscores. This was chosen
over rejecting underscores in `hostname` when the config loads, which
would also stop restores on such a host.

The purge consistency test stored a hostname that did not match its
snapshot IDs; it now matches, as it always does in production.

Model: opus-5-5

TODO.md

@@ -22,6 +22,17 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-07: Made per-name retention work when the hostname contains `_`
+  ([issue #230](https://git.eeqj.de/sneak/vaultik/issues/230)). A
+  snapshot ID is `hostname_name_timestamp`, and the name was read as
+  everything between the first and the last `_`, so with
+  `hostname: my_host` the name `home` came out as `host_home`.
+  `snapshot purge --keep-latest --snapshot home` then printed "No
+  snapshots to delete", and `snapshot create --prune` purged nothing
+  without a message. The name is now read using the hostname the
+  `snapshots` table stores with each snapshot, cut at its first `.` as it
+  is in the ID.
+
 - 2026-10-07: Made `remote info` stop reporting a snapshot's blobs as
   orphaned when its manifest cannot be read, and stop printing raw
   names from under `metadata/`