fix(mcp-image-gen): correct ComfyUI install instructions in USAGE.md

ComfyUI is NOT on PyPI — `pip install comfyui` fails with
"No matching distribution found". Remove the wrong Option A.

Replace with:
- Warning note that pip install does not work
- Only correct method: git clone from GitHub + pip install -r requirements.txt

ROCm status confirmed: rocm-smi 3.1.0 / ROCm-SMI-LIB 7.7.0 installed.

mcp/mcp-image-gen/USAGE.md

@@ -27,11 +27,11 @@ The MCP server connects to ComfyUI's REST API at `http://localhost:8188`. If Com
 
 ### Install ComfyUI
 
-```bash
-# Option A — pip install (simplest)
-pip install comfyui
+> ⚠️ **ComfyUI is NOT on PyPI** — `pip install comfyui` will fail with "No matching distribution found".
+> It must be installed from source via `git clone`.
 
-# Option B — git clone (more control)
+```bash
+# Clone from source (the only correct installation method)
 git clone https://github.com/comfyanonymous/ComfyUI.git
 cd ComfyUI
 pip install -r requirements.txt