refactor: extract torch install helper and unify PR reference parsers

[bigcat88]

The torch installation in pip_install_comfyui_dependencies repeated the same subprocess.run([python, "-m", "pip", "install", "torch", "torchvision", "torchaudio"] + index_args) pattern four times for AMD Linux, NVIDIA, Intel Arc, and CPU backends. Extracted a _pip_install_torch helper that all four call. DirectML and Mac M Series have different command shapes so they stay inline.

parse_pr_reference and parse_frontend_pr_reference were copy-pasted functions that only differed in the default repo (comfyanonymous/ComfyUI vs Comfy-Org/ComfyUI_frontend). Unified them into a shared _parse_pr_reference(pr_ref, default_owner, default_repo) with thin public wrappers. Also hardcoded the upstream repo in handle_temporary_frontend_pr's find_pr_by_branch call to match how handle_pr_checkout already works.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
comfy_cli/command/install.py	75.00%	4 Missing ⚠️

@@            Coverage Diff             @@
##             main     #389      +/-   ##
==========================================
- Coverage   71.07%   70.98%   -0.10%     
==========================================
  Files          33       33              
  Lines        3931     3915      -16     
==========================================
- Hits         2794     2779      -15     
+ Misses       1137     1136       -1     

Files with missing lines	Coverage Δ
comfy_cli/command/install.py	75.59% <75.00%> (-0.56%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.