fix: use --index-url for AMD ROCm torch install, add --rocm-version flag

[bigcat88]

comfy install --amd installs CUDA torch instead of ROCm torch. The root cause is that --extra-index-url lets pip see both PyPI and the ROCm index simultaneously, and pip picks the higher-versioned CUDA torch from PyPI over the ROCm build.

This switches to --index-url (which replaces PyPI entirely) following PyTorch's official recommendation. Also adds a --rocm-version flag mirroring the existing --cuda-version, so users with older ROCm drivers can select their version. Default is rocm6.3.

The uv fast-deps path is also bumped from rocm6.1 to rocm6.3 for consistency.

Fixes #289

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #379      +/-   ##
==========================================
+ Coverage   62.99%   63.10%   +0.11%     
==========================================
  Files          33       33              
  Lines        3691     3697       +6     
==========================================
+ Hits         2325     2333       +8     
+ Misses       1366     1364       -2     

Files with missing lines	Coverage Δ
comfy_cli/cmdline.py	58.00% <100.00%> (ø)
comfy_cli/command/install.py	70.86% <100.00%> (+0.39%)	⬆️
comfy_cli/constants.py	100.00% <100.00%> (ø)
comfy_cli/uv.py	55.89% <100.00%> (ø)

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