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/snapshot.go

@@ -97,7 +97,10 @@ func (v *Vaultik) CreateSnapshot(opts *SnapshotCreateOptions) error {
 		log.Info("Pruning enabled - deleting old snapshots and unreferenced blobs")
 		v.printlnStdout("\nPruning old snapshots (keeping latest)...")
 
-		if err := v.PurgeSnapshots(true, "", true); err != nil {
+		if err := v.PurgeSnapshotsWithOptions(&SnapshotPurgeOptions{
+			KeepLatest: true,
+			Force:      true,
+		}); err != nil {
 			return fmt.Errorf("prune: purging old snapshots: %w", err)
 		}
 
@@ -582,8 +585,19 @@ func (v *Vaultik) printSnapshotTable(snapshots []SnapshotInfo) error {
 	return w.Flush()
 }
 
-// PurgeSnapshots removes old snapshots based on criteria
-func (v *Vaultik) PurgeSnapshots(keepLatest bool, olderThan string, force bool) error {
+// SnapshotPurgeOptions contains options for the snapshot purge command
+type SnapshotPurgeOptions struct {
+	KeepLatest bool
+	OlderThan  string
+	Force      bool
+	Name       string // Filter purge to a specific snapshot name
+}
+
+// PurgeSnapshotsWithOptions removes old snapshots based on criteria.
+// When KeepLatest is true, retention is applied per snapshot name — the latest
+// snapshot for each distinct name is kept. If Name is non-empty, only snapshots
+// matching that name are considered for purge.
+func (v *Vaultik) PurgeSnapshotsWithOptions(opts *SnapshotPurgeOptions) error {
 	// Sync with remote first
 	if err := v.syncWithRemote(); err != nil {
 		return fmt.Errorf("syncing with remote: %w", err)
@@ -607,14 +621,51 @@ func (v *Vaultik) PurgeSnapshots(keepLatest bool, olderThan string, force bool)
 		}
 	}
 
+	// If --name is specified, filter to only snapshots matching that name
+	if opts.Name != "" {
+		filtered := make([]SnapshotInfo, 0, len(snapshots))
+		for _, snap := range snapshots {
+			if parseSnapshotName(snap.ID.String()) == opts.Name {
+				filtered = append(filtered, snap)
+			}
+		}
+		snapshots = filtered
+	}
+
 	// Sort by timestamp (newest first)
 	sort.Slice(snapshots, func(i, j int) bool {
 		return snapshots[i].Timestamp.After(snapshots[j].Timestamp)
 	})
 
-	toDelete, err := v.collectSnapshotsToPurge(snapshots, keepLatest, olderThan)
-	if err != nil {
-		return err
+	var toDelete []SnapshotInfo
+
+	if opts.KeepLatest {
+		// Keep the latest snapshot per snapshot name
+		// Group snapshots by name, then mark all but the newest in each group
+		latestByName := make(map[string]bool) // tracks whether we've seen the latest for each name
+		for _, snap := range snapshots {
+			name := parseSnapshotName(snap.ID.String())
+			if latestByName[name] {
+				// Already kept the latest for this name — delete this one
+				toDelete = append(toDelete, snap)
+			} else {
+				// This is the latest (sorted newest-first) — keep it
+				latestByName[name] = true
+			}
+		}
+	} else if opts.OlderThan != "" {
+		// Parse duration
+		duration, err := parseDuration(opts.OlderThan)
+		if err != nil {
+			return fmt.Errorf("invalid duration: %w", err)
+		}
+
+		cutoff := time.Now().UTC().Add(-duration)
+		for _, snap := range snapshots {
+			if snap.Timestamp.Before(cutoff) {
+				toDelete = append(toDelete, snap)
+			}
+		}
 	}
 
 	if len(toDelete) == 0 {
@@ -622,37 +673,7 @@ func (v *Vaultik) PurgeSnapshots(keepLatest bool, olderThan string, force bool)
 		return nil
 	}
 
-	return v.confirmAndExecutePurge(toDelete, force)
-}
-
-// collectSnapshotsToPurge determines which snapshots to delete based on retention criteria
-func (v *Vaultik) collectSnapshotsToPurge(snapshots []SnapshotInfo, keepLatest bool, olderThan string) ([]SnapshotInfo, error) {
-	if keepLatest {
-		// Keep only the most recent snapshot
-		if len(snapshots) > 1 {
-			return snapshots[1:], nil
-		}
-		return nil, nil
-	}
-
-	if olderThan != "" {
-		// Parse duration
-		duration, err := parseDuration(olderThan)
-		if err != nil {
-			return nil, fmt.Errorf("invalid duration: %w", err)
-		}
-
-		cutoff := time.Now().UTC().Add(-duration)
-		var toDelete []SnapshotInfo
-		for _, snap := range snapshots {
-			if snap.Timestamp.Before(cutoff) {
-				toDelete = append(toDelete, snap)
-			}
-		}
-		return toDelete, nil
-	}
-
-	return nil, nil
+	return v.confirmAndExecutePurge(toDelete, opts.Force)
 }
 
 // confirmAndExecutePurge shows deletion candidates, confirms with user, and deletes snapshots