feat: add pygit2 compatibility wrapper for standalone Desktop 2.0 installs (#2719)

* feat: add pygit2 compatibility wrapper for standalone Desktop 2.0 installs

Add git_compat.py abstraction layer that wraps both GitPython and pygit2
behind a unified GitRepo interface. When CM_USE_PYGIT2=1 is set (by the
Desktop 2.0 Launcher for standalone installs), or when system git is not
available, the pygit2 backend is used automatically.

Key changes:
- New comfyui_manager/common/git_compat.py with abstract GitRepo base class,
  _GitPythonRepo (1:1 pass-throughs) and _Pygit2Repo implementations
- All 6 non-legacy files updated to use the wrapper:
  - comfyui_manager/glob/manager_core.py (14 git.Repo usages)
  - comfyui_manager/common/git_helper.py (7 git.Repo usages)
  - comfyui_manager/common/context.py (1 usage)
  - comfyui_manager/glob/utils/environment_utils.py (2 usages)
  - cm_cli/__main__.py (1 usage)
  - comfyui_manager/common/timestamp_utils.py (repo.heads usage)
- get_script_env() propagates CM_USE_PYGIT2 to subprocesses
- git_helper.py uses sys.path.insert to find git_compat as standalone script
- All repo handles wrapped in context managers to prevent resource leaks
- get_head_by_name returns _HeadProxy for interface consistency
- Submodule fallback logs clear message when system git is absent
- 47 tests comparing both backends via subprocess isolation

Co-authored-by: Amp <amp@ampcode.com>
Amp-Thread-ID: https://ampcode.com/threads/T-019d0ec5-cb9f-74df-a1a2-0c8154a330b3

* fix(pygit2): address review findings + bump version to 4.2b1

- C1: add submodule_update() after pygit2 clone for recursive parity
- C2: check subprocess returncode in submodule_update fallback
- C3: move GIT_OPT_SET_OWNER_VALIDATION to module-level (once at import)
- H1: use context manager in git_pull() to prevent resource leaks
- Bump version to 4.2b1, version_code to [4, 2]
- Add pygit2 to dev dependencies and requirements.txt

* style: fix ruff F841 unused variable and F541 unnecessary f-string

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: Dr.Lt.Data <dr.lt.data@gmail.com>

pyproject.toml

@@ -5,7 +5,7 @@ build-backend = "setuptools.build_meta"
 [project]
 name = "comfyui-manager"
 license = { text = "GPL-3.0-only" }
-version = "4.1"
+version = "4.2b1"
 requires-python = ">= 3.9"
 description = "ComfyUI-Manager provides features to install and manage custom nodes for ComfyUI, as well as various functionalities to assist with ComfyUI."
 readme = "README.md"
@@ -39,7 +39,7 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
-dev = ["pre-commit", "pytest", "ruff", "pytest-cov"]
+dev = ["pre-commit", "pytest", "ruff", "pytest-cov", "pygit2"]
 
 [project.urls]
 Repository = "https://github.com/ltdrdata/ComfyUI-Manager"