feat: per-name purge filtering for snapshot purge (#51)

## Summary

`PurgeSnapshots` now applies `--keep-latest` retention per snapshot name instead of globally across all names.

### Problem

Previously, `--keep-latest` would keep only the single most recent snapshot across ALL snapshot names. For example, with snapshots:
- `system_2024-01-15`
- `home_2024-01-14`
- `system_2024-01-13`

`--keep-latest` would keep only `system_2024-01-15` and delete the latest `home` snapshot too.

### Solution

1. **Per-name retention**: `--keep-latest` now groups snapshots by name and keeps the latest of each group. In the example above, both `system_2024-01-15` and `home_2024-01-14` would be kept.

2. **`--name` flag**: New flag to filter purge operations to a specific snapshot name. `--name home --keep-latest` only purges `home` snapshots, leaving all `system` snapshots untouched.

### Changes

- `internal/vaultik/helpers.go`: Add `parseSnapshotName()` to extract the snapshot name from a snapshot ID (`hostname_name_timestamp` format)
- `internal/vaultik/snapshot.go`: Add `SnapshotPurgeOptions` struct with `Name` field, add `PurgeSnapshotsWithOptions()` method, modify `--keep-latest` logic to group by name
- `internal/cli/purge.go` and `internal/cli/snapshot.go`: Add `--name` flag to both purge CLI surfaces
- `README.md`: Update CLI documentation

### Tests

- `helpers_test.go`: Unit tests for `parseSnapshotName()` and `parseSnapshotTimestamp()`
- `purge_per_name_test.go`: Integration tests covering:
  - Per-name retention with multiple names
  - Single-name retention
  - `--name` filter with `--keep-latest`
  - `--name` filter with `--older-than`
  - No-match name filter (all snapshots retained)
  - Legacy snapshots without name component
  - Mixed named and legacy snapshots
  - Three different snapshot names

### Backward Compatibility

The existing `PurgeSnapshots(keepLatest, olderThan, force)` signature is preserved as a wrapper around the new `PurgeSnapshotsWithOptions()`. The `--prune` flag in `snapshot create` continues to work unchanged.

`docker build .` passes (lint, fmt-check, all tests).

closes [#9](#9)

Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #51
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/vaultik/helpers_test.go

@@ -0,0 +1,76 @@
+package vaultik
+
+import (
+	"testing"
+)
+
+func TestParseSnapshotName(t *testing.T) {
+	tests := []struct {
+		name       string
+		snapshotID string
+		want       string
+	}{
+		{
+			name:       "standard format with name",
+			snapshotID: "myhost_home_2026-01-12T14:41:15Z",
+			want:       "home",
+		},
+		{
+			name:       "standard format with different name",
+			snapshotID: "server1_system_2026-02-15T09:30:00Z",
+			want:       "system",
+		},
+		{
+			name:       "name with underscores",
+			snapshotID: "myhost_my_special_backup_2026-03-01T00:00:00Z",
+			want:       "my_special_backup",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := parseSnapshotName(tt.snapshotID)
+			if got != tt.want {
+				t.Errorf("parseSnapshotName(%q) = %q, want %q", tt.snapshotID, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestParseSnapshotTimestamp(t *testing.T) {
+	tests := []struct {
+		name       string
+		snapshotID string
+		wantErr    bool
+	}{
+		{
+			name:       "valid with name",
+			snapshotID: "myhost_home_2026-01-12T14:41:15Z",
+			wantErr:    false,
+		},
+		{
+			name:       "valid without name",
+			snapshotID: "myhost_2026-01-12T14:41:15Z",
+			wantErr:    false,
+		},
+		{
+			name:       "invalid - single part",
+			snapshotID: "nounderscore",
+			wantErr:    true,
+		},
+		{
+			name:       "invalid - bad timestamp",
+			snapshotID: "myhost_home_notadate",
+			wantErr:    true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := parseSnapshotTimestamp(tt.snapshotID)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("parseSnapshotTimestamp(%q) error = %v, wantErr %v", tt.snapshotID, err, tt.wantErr)
+			}
+		})
+	}
+}