feat: add ComfyUI symlink functionality and subcommands

Adds subcommand structure (download, link, both) to the ComfyUI downloader script, allowing users to download models from HuggingFace and/or create symlinks to ComfyUI model directories. Features include:

- New subcommands: download (download only), link (symlink only), both (default)
- ComfyUI directory configuration (--comfyui-dir, default: ~/ComfyUI/models)
- Smart symlink creation to appropriate ComfyUI subdirectories (checkpoints, vae, loras, controlnet, upscale_models, etc.)
- YAML configuration extended with 'type' and 'filename' fields for precise model organization
- Automatic ComfyUI subdirectory creation
- Graceful handling of existing symlinks and files
- HF_TOKEN validation only when needed (download/both commands)
- Example configuration file (comfyui_models.example.yaml) demonstrating proper setup

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

comfyui_models.example.yaml

@@ -0,0 +1,80 @@
+# ComfyUI Models Configuration Example
+#
+# This file defines which models to download from HuggingFace
+# and where to symlink them in your ComfyUI installation.
+#
+# Model types correspond to ComfyUI subdirectories:
+#   - checkpoints: Stable Diffusion checkpoints
+#   - vae: VAE models
+#   - loras: LoRA models
+#   - controlnet: ControlNet models
+#   - clip: CLIP models
+#   - clip_vision: CLIP Vision models
+#   - upscale_models: Upscaler models
+#   - embeddings: Textual inversion embeddings
+#   - hypernetworks: Hypernetwork models
+#   - style_models: Style transfer models
+
+settings:
+  cache_dir: ~/.cache/huggingface
+  parallel_downloads: 1
+
+model_categories:
+  # Stable Diffusion Checkpoints
+  checkpoints:
+    - repo_id: runwayml/stable-diffusion-v1-5
+      description: Stable Diffusion v1.5
+      size_gb: 4
+      essential: true
+      type: checkpoints
+      filename: ""  # Empty means all model files
+
+    - repo_id: stabilityai/stable-diffusion-xl-base-1.0
+      description: SDXL Base 1.0
+      size_gb: 7
+      essential: true
+      type: checkpoints
+      filename: ""
+
+  # VAE Models
+  vae:
+    - repo_id: stabilityai/sd-vae-ft-mse-original
+      description: SD VAE ft MSE
+      size_gb: 0.3
+      essential: true
+      type: vae
+      filename: ""
+
+  # LoRA Models
+  loras:
+    - repo_id: latent-consistency/lcm-lora-sdv1-5
+      description: LCM LoRA for SD v1.5
+      size_gb: 0.1
+      essential: false
+      type: loras
+      filename: ""
+
+  # ControlNet Models
+  controlnet:
+    - repo_id: lllyasviel/control_v11p_sd15_canny
+      description: ControlNet Canny
+      size_gb: 1.4
+      essential: false
+      type: controlnet
+      filename: ""
+
+    - repo_id: lllyasviel/control_v11p_sd15_openpose
+      description: ControlNet OpenPose
+      size_gb: 1.4
+      essential: false
+      type: controlnet
+      filename: ""
+
+  # Upscale Models
+  upscale_models:
+    - repo_id: ai-forever/Real-ESRGAN
+      description: Real-ESRGAN x4
+      size_gb: 0.1
+      essential: false
+      type: upscale_models
+      filename: "RealESRGAN_x4plus.pth"