[DGX Spark][Install] Installer completes without checking required host python3, leaving Hermes rebuild unusable

## Description

A supported NemoClaw installation and Hermes onboarding can complete on a host that does not have a verified `python3` interpreter at any accepted location. The missing prerequisite is discovered only later when a normal Hermes rebuild reaches snapshot sanitization and exits 1.

The public Prerequisites page identifies a verified host `python3` interpreter as required for snapshot sanitization, but the installer does not validate this prerequisite before reporting setup success.

Platform scope: Reproduced on DGX Spark with Ubuntu 24.04 only; other platforms not tested
Regression: Unknown — earlier versions not tested
OpenShell issue: No

## Environment

```text
Device: NVIDIA DGX Spark / GB10
OS: Ubuntu 24.04.4 LTS (Linux 7.0.0-1010-nvidia)
Architecture: aarch64
Node.js: v22.23.2
npm: 10.9.8
Docker: Docker version 29.2.1, build a5c7197
OpenShell CLI: openshell 0.0.106
NemoClaw: v0.0.119
OpenClaw: N/A (Hermes sandbox)
```

## Steps to Reproduce

1. On a supported DGX Spark host, confirm that no verified `python3` interpreter exists at any location accepted by NemoClaw snapshot sanitization.
2. Install NemoClaw v0.0.119 with the official installer and complete normal Hermes onboarding for a fresh sandbox named `docval-hermes-rebuild`.
3. Run:

```bash
nemohermes docval-hermes-rebuild rebuild --yes
```

4. Check the command exit code and the sandbox state.

## Expected Result

The installer should validate every host prerequisite needed by supported lifecycle operations before reporting setup success. It should either provision a supported interpreter or stop early with actionable remediation. After a successful supported setup, Hermes rebuild should complete.

## Actual Result

The installation and Hermes onboarding completed, but rebuild failed during its backup phase:

```text
Backing up sandbox state...
Error: python3 is required for snapshot sanitization; incomplete backup removed.
```

The rebuild command exited with status 1.

## Logs

```text
Command: nemohermes docval-hermes-rebuild rebuild --yes
Exit code: 1

Backing up sandbox state...
Error: python3 is required for snapshot sanitization; incomplete backup removed.
```