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.go

@@ -79,6 +79,22 @@ func parseSnapshotTimestamp(snapshotID string) (time.Time, error) {
 	return timestamp.UTC(), nil
 }
 
+// parseSnapshotName extracts the snapshot name from a snapshot ID.
+// Format: hostname_snapshotname_timestamp — the middle part(s) between hostname
+// and the RFC3339 timestamp are the snapshot name (may contain underscores).
+// Returns the snapshot name, or empty string if the ID is malformed.
+func parseSnapshotName(snapshotID string) string {
+	parts := strings.Split(snapshotID, "_")
+	if len(parts) < 3 {
+		// Format: hostname_timestamp — no snapshot name
+		return ""
+	}
+	// Format: hostname_name_timestamp — middle parts are the name.
+	// The last part is the RFC3339 timestamp, the first part is the hostname,
+	// everything in between is the snapshot name (which may itself contain underscores).
+	return strings.Join(parts[1:len(parts)-1], "_")
+}
+
 // parseDuration parses a duration string with support for days
 func parseDuration(s string) (time.Duration, error) {
 	// Check for days suffix