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>

README.md

@@ -150,7 +150,7 @@ passphrase is needed or stored locally.
 vaultik [--config <path>] snapshot create [snapshot-names...] [--cron] [--daemon] [--prune]
 vaultik [--config <path>] snapshot list [--json]
 vaultik [--config <path>] snapshot verify <snapshot-id> [--deep]
-vaultik [--config <path>] snapshot purge [--keep-latest | --older-than <duration>] [--force]
+vaultik [--config <path>] snapshot purge [--keep-latest | --older-than <duration>] [--name <name>] [--force]
 vaultik [--config <path>] snapshot remove <snapshot-id> [--dry-run] [--force]
 vaultik [--config <path>] snapshot prune
 vaultik [--config <path>] restore <snapshot-id> <target-dir> [paths...]
@@ -180,8 +180,9 @@ vaultik [--config <path>] store info
 * `--deep`: Download and verify blob contents (not just existence)
 
 **snapshot purge**: Remove old snapshots based on criteria
-* `--keep-latest`: Keep only the most recent snapshot
+* `--keep-latest`: Keep the most recent snapshot per snapshot name
 * `--older-than`: Remove snapshots older than duration (e.g., 30d, 6mo, 1y)
+* `--name`: Filter purge to a specific snapshot name
 * `--force`: Skip confirmation prompt
 
 **snapshot remove**: Remove a specific snapshot