feat(types): validate snapshot name length and shape

[CMGS]

Summary

`cocoon snapshot save`/`import` previously accepted any string for `--name` — no length cap, no charset check. A snapshot named with 100+ chars or shell-unsafe chars would write straight into the DB, the OCI annotation on push, and any downstream propagation (Linux `HOST_NAME_MAX=64`, K8s DNS-1123 labels in pod name concatenation, etc).

This PR closes the gap by mirroring `VMConfig.Validate`'s rule on `SnapshotConfig`:

```go
validName = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9._-]{0,62}$`)
```

(1 leading char + ≤62 trailing = max 63 chars, fitting under Linux's HOST_NAME_MAX boundary.)

Changes

File	Change
`types/snapshot.go`	New `SnapshotConfig.Validate()` method (empty Name still allowed)
`types/snapshot_test.go`	9 subtests covering empty / valid / over-63 / leading-hyphen / space / slash / control char
`cmd/snapshot/handler.go`	`Save` + `Import` validate the `--name` flag early — fail before expensive snapshot/import work
`snapshot/localfile/localfile.go`	`Create` runs Validate before insert (defense-in-depth)
`snapshot/localfile/import.go`	`Import` runs Validate after envelope/override merge

Behavior

Before:
```
$ cocoon snapshot save vm --name "my-very-very-long-name-that-keeps-going-and-going-and-blows-past-63-chars"
snapshot saved: ULID... # silently OK, downstream breaks later
```

After:
```
$ cocoon snapshot save vm --name "my-very-very-long-name-...-blows-past-63-chars"
Error: snapshot name "my-very..." is invalid: must match ^[a-zA-Z0-9][a-zA-Z0-9._-]{0,62}$ (max 63 chars)
```

Same for `cocoon snapshot import --name X`.

Empty `--name` is still allowed (name is optional, ID is the fallback identifier).

Why

Reported by @doge: CH-bound hostname (set via cloud-init cidata or kernel cmdline) hits Linux `HOST_NAME_MAX=64` boundary. When vk-cocoon concatenates pod names → `cocoon snapshot save --name `, an overly long name would have been silently accepted, then surface as a downstream failure. `VMConfig` already validated this; SnapshotConfig didn't.

Verification

```
✓ go build ./...
✓ make lint (0 issues × Linux + Darwin)
✓ go test ./... (all green; 9 new SnapshotConfig.Validate subtests pass)
```

Test plan

• `cocoon snapshot save vm --name foo` works
• `cocoon snapshot save vm --name $(python -c 'print("a"*64)')` rejected with clear error
• `cocoon snapshot save vm --name 'has space'` rejected
• `cocoon snapshot save vm` (no `--name`) works (empty allowed)
• `cocoon snapshot import file.tar.gz --name bad$char` rejected before opening file