#914 strata-vision helper leaks /tmp/strata-vision-* temp dirs (unbounded growth -> OOM on tmpfs /tmp)

Description

On v0.1.38 (CUDA 13.4 build, RTX 3090 Ti, CachyOS), `strata-vision` creates a fresh `mkdtemp` directory `/tmp/strata-vision-<rand>/` for every vision request, filled with `.sve` scratch files (~150-200 MB per dir), and never removes them - neither after the request nor on helper exit.

Observed after ~1 day of normal native-vision traffic: **120 leaked dirs ~ 12 GB**. On distros where `/tmp` is tmpfs (CachyOS, Fedora defaults), this consumes physical RAM directly and pushes the overflow into zram, which is also RAM - our 62 GiB box hit 62/62 GiB used with 420 MB available and nearly OOMed.

Repro: run the vision path repeatedly (`--vision --vram-reserve-mib 700`, OpenAI-style `image_url` requests), watch `ls -d /tmp/strata-vision-* | wc -l` grow monotonically; each dir contains multiple ~10 MB `.sve` files.

Expected: helper unlinks its temp dirs after each request (or at least on clean exit), or reuses a single scratch dir per helper process.

Workaround we run meanwhile: `0 */6 * * * find /tmp -maxdepth 1 -name 'strata-vision-*' -type d -mmin +360 -exec rm -rf {} +`