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

internal/snapshot/snapshot.go

@@ -105,17 +105,21 @@ func (sm *SnapshotManager) CreateSnapshot(
 	return sm.CreateSnapshotWithName(ctx, hostname, "", version, gitRevision)
 }
 
+// ShortHostname returns hostname up to its first dot. A snapshot ID starts
+// with this form, while the snapshots table stores the full hostname.
+func ShortHostname(hostname string) string {
+	short, _, _ := strings.Cut(hostname, ".")
+
+	return short
+}
+
 // CreateSnapshotWithName creates a new snapshot record with an optional
 // snapshot name. The snapshot ID format is: hostname_name_timestamp or
 // hostname_timestamp if name is empty.
 func (sm *SnapshotManager) CreateSnapshotWithName(
 	ctx context.Context, hostname, name, version, gitRevision string,
 ) (string, error) {
-	// Use short hostname (strip domain if present)
-	shortHostname := hostname
-	if before, _, ok := strings.Cut(hostname, "."); ok {
-		shortHostname = before
-	}
+	shortHostname := ShortHostname(hostname)
 
 	// Build snapshot ID with optional name
 	timestamp := time.Now().UTC().Format("2006-01-02T15:04:05Z")