[Ubuntu 24.04][Sandbox] snapshot restore does not remove files added after the snapshot — regression of #1902

> [!WARNING]
> **Regression** — this bug was fixed in #1902 (April 2026), but has resurfaced in v0.0.76.

## Description

`nemoclaw snapshot restore` succeeds (exit 0, reports "Restored 14 directories, 1 files") but does not delete files that were added to the sandbox after the snapshot was created. A file written to `/sandbox/.hermes-data/workspace/` after snapshot creation survives the restore and is still present afterwards. This is a regression of the fix for #1902, which corrected the same tar-overlay semantics problem.

Platform scope: Reproduced on Ubuntu 24.04 (ubuntu22 and ubuntu24 CI runners); DGX Spark hit a different error on the same test. Other platforms not tested.

## Environment

```text
Device: Ubuntu 24.04 server (x86_64 CI runner)
OS: Ubuntu 24.04
Architecture: x86_64
Node.js: v22.22.3
npm: 10.9.8
Docker: 28.3.3
OpenShell CLI: 0.0.72
NemoClaw: v0.0.76
OpenClaw: 2026.6.10
```

## Steps to Reproduce

1. `nemoclaw onboard --name test-sandbox --non-interactive --yes`
2. `nemoclaw test-sandbox snapshot create --name baseline`
3. `nemoclaw test-sandbox connect`
4. Inside sandbox: `echo "stale content" > /sandbox/.hermes-data/workspace/stale-file.txt`
5. `exit`
6. `nemoclaw test-sandbox snapshot restore baseline --yes`
7. `nemoclaw test-sandbox connect`
8. Inside sandbox: `ls /sandbox/.hermes-data/workspace/stale-file.txt`

## Expected Result

Step 8: `stale-file.txt` should not exist — snapshot restore must revert the sandbox to the exact state at snapshot creation time, removing any files added afterwards.

## Actual Result

Step 6 output:
```text
Using snapshot v1 (2026-07-08T02-41-26-204Z)
Restoring snapshot into 'test-sandbox'...
Restored 14 directories, 1 files
OpenClaw config permissions restored
```

Step 8: `stale-file.txt` is PRESENT — the file survived the restore. The restore used tar overlay semantics (only overwrites files in the archive) instead of clean-state semantics (delete everything, then extract).

## Logs

```text
$ nemoclaw test-sandbox snapshot restore baseline --yes
Using snapshot v1 (2026-07-08T02-41-26-204Z)
Restoring snapshot into 'test-sandbox'...
Restored 14 directories, 1 files
OpenClaw config permissions restored

$ nemoclaw test-sandbox connect
sandbox@test-sandbox:~$ cat /sandbox/.hermes-data/workspace/stale-file.txt
stale content
```

---
[NVB#6428493](https://nvbugspro.nvidia.com/bug/6428493)