#1322 setup re-run with --vision gpu does not add --vision to an existing config's args

Descrição

## What happened

On a machine whose `strata-*.json` was created earlier without images, re-running setup to enable vision:

```bash
./setup.sh --setup --model IQ3_S --vision gpu --context 262144 --yes
```

writes the `vision` section into the config but leaves `args` without `--vision --vram-reserve-mib 700`. The server then starts the engine without vision and every image request fails with:

> `this engine was started without --vision`

while `/v1/models` still advertises `input_modalities: ["text", "image"]` (taken from the vision section), which is misleading.

## Workaround

Manually append `--vision --vram-reserve-mib 700` to the config's `args` and restart. Verified working on two nodes (RTX 5090, RTX PRO 6000).

## Suspected cause

In `setup.py`, `args += ["--vision", "--vram-reserve-mib", ...]` only runs on the fresh-config path. The re-run path preserves the existing `args` and only adds the `vision` section, so the engine never gets the flag.

## Environment

- Strata at commit `82f46a8` (engine 0.1.38 / 0.1.40, local build, CUDA 13.2, sm_120)
- Ubuntu 24.04, RTX 5090 32GB / RTX PRO 6000 96GB
- Qwen3.8-Flash-Next IQ3_S