Update README models table: 2 shipped + community alternatives

[FuJacob]

Summary

The Open Source engine catalog dropped to two models in #251 (tabby-fast-1 and tabby-balanced-1), but the README still listed four rows under the pre-rename cotabby-* display names. Replace the stale table with the current two shipped models (with Hugging Face source links) and add a Bring your own model subsection so users know the menu-bar refresh path picks up any GGUF dropped into the models folder. The alternatives table lists two GGUFs Cotabby has shipped previously (Qwen3.5-0.8B, Gemma 3 1B), with a link to the unsloth org for browsing more variants.

Validation

curl -s -o /dev/null -w '%{http_code}' -L https://huggingface.co/unsloth/Qwen3-0.6B-GGUF       # 200
curl -s -o /dev/null -w '%{http_code}' -L https://huggingface.co/unsloth/gemma-4-E2B-it-GGUF   # 200
curl -s -o /dev/null -w '%{http_code}' -L https://huggingface.co/unsloth/Qwen3.5-0.8B-GGUF     # 200
curl -s -o /dev/null -w '%{http_code}' -L https://huggingface.co/unsloth/gemma-3-1b-it-GGUF    # 200
curl -s -o /dev/null -w '%{http_code}' -L https://huggingface.co/unsloth                       # 200

Docs-only change; no code paths touched.

Linked issues

None.

Risk / rollout notes

• Display-name change (cotabby-* → tabby-*) matches what the menu bar actually shows today (RuntimeModelCatalog.displayName), so users hitting the README from a fresh install won't see a mismatch anymore.
• External links to Hugging Face — verified live at PR time but obviously can rot. Cheap to fix if they do.