Add CI model checks and fixes "arch" field in config

[andriiryzhkov]

1. CI model checks — Adds a check-models workflow that runs the full pipeline (setup, convert, validate, demo) per non-skipped model as individual GitHub Actions checks. Updates build-models to use the same per-model dependency isolation. CLI now auto-syncs uv dependency groups and auto-initializes submodules, so uv run dtai run <model> works out of the box.

2. arch field in config — Adds arch (default "generic") to ModelConfig and writes it to config.json, used by darktable's C-side inference pipeline for architecture-specific dispatch (e.g. sam2, segnext).

[TurboGit]

Thanks!