Closes#132 — jarvis init now recommends a Qwen3 dense model based on
available memory using an explicit tier table:
≤8 GB → qwen3:1.7b (~1.1 GB)
8-16GB → qwen3:4b (~2.5 GB)
16-32GB → qwen3:8b (~5.2 GB)
32GB+ → qwen3:14b (~9.3 GB)
Adds qwen3:0.6b, 1.7b, 4b, 14b, 30b to the model catalog. Falls back
to scanning all compatible models if the tiered pick doesn't support
the selected engine. Shows the user why the model was chosen and how
to download later if declined.
