[Ubuntu 26.04][Sandbox] force rebuild still loses mounted workspace contents after GitHub #10639 fix

> [!WARNING]
> **Regression** — this same symptom was reported and closed as fixed via #10639, but reproduces again on the current release.

## Description

Running `nemoclaw {sandbox} rebuild --force --yes` still discards a file written into the sandbox workspace before rebuild, reproducing the identical symptom reported and closed as fixed in #10639. Re-verification was performed specifically because that issue's disposition was Bug-Fixed; the fix (PR merged 2026-08-31) changed rebuild to fail closed whenever a manifest-declared state directory or file cannot be backed up, but the workspace mount itself does not appear to be included in whatever the rebuild pipeline currently treats as a preservable state target — rebuild completes successfully (exit 0, "State restored") while the workspace marker is still gone afterward.

Platform scope: Reproduced on Ubuntu 26.04 only; other platforms not tested for this re-verification pass.
Regression: Yes — #10639 reported this same symptom, was closed as fixed by a merged PR, but the identical symptom reproduces again on the current release.
OpenShell issue: No — the failing surface is NemoClaw's own rebuild/state-restoration path.

## Environment

```text
Device: Linux server (Docker container runtime)
OS: Ubuntu 26.04 LTS
Architecture: x86_64
Node.js: v22.23.2
npm: 10.9.8
Docker: 29.7.2
OpenShell CLI: 0.0.106
NemoClaw: v0.0.119
OpenClaw: 2026.7.1
```

## Steps to Reproduce

1. `nemoclaw onboard --fresh --name {sandbox} --non-interactive --yes --yes-i-accept-third-party-software`
2. `nemoclaw {sandbox} exec -- sh -c 'echo FORCE_REBUILD_MARKER > /sandbox/workspace-marker.txt; cat /sandbox/workspace-marker.txt'`
3. `nemoclaw {sandbox} rebuild --force --yes`
4. `nemoclaw {sandbox} exec -- cat /sandbox/workspace-marker.txt`

## Expected Result

Per the documented rebuild contract (and per #10639's expected result), the force rebuild completes with the sandbox usable, and the workspace marker still contains the original marker content.

## Actual Result

Step 3 (rebuild) exits 0 and reports "State restored (11 directories, 1 files)". Step 4 fails:

```text
cat: /sandbox/workspace-marker.txt: No such file or directory
```

A directory listing of `/sandbox` after rebuild shows only `.nemoclaw`, `.openclaw`, `.profile`, and `.bashrc` — the workspace marker file is gone, and no other top-level file exists in its place. This is the exact symptom #10639 reported ("Received workspace marker content: MISSING"), reproduced on the current release after that issue's fix was merged.

## Logs

```text
Restoring workspace state...
✓ State restored (11 directories, 1 files)
✓ Post-upgrade structure check passed
✓ Mutable config permissions restored

✓ Sandbox '{sandbox}' rebuild completed
Now running: OpenClaw v2026.7.1

$ nemoclaw {sandbox} exec -- cat /sandbox/workspace-marker.txt
cat: /sandbox/workspace-marker.txt: No such file or directory
```

Start with the `nemoclaw {sandbox} rebuild --force --yes` entry point and trace the state backup and restoration path described in #10639. Reproduce the four commands on Ubuntu 26.04, then verify that a file created in `/sandbox` survives a force rebuild and that the reported restored-state counts remain accurate.

Written by the indexing model from the issue text.