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>

cm_cli/__main__.py

@@ -11,7 +11,6 @@ import typer
 from rich import print
 from typing_extensions import List, Annotated
 import re
-import git
 import importlib
 
 
@@ -62,9 +61,10 @@ if os.path.exists(os.path.join(manager_util.comfyui_manager_path, "pip_blacklist
 
 def check_comfyui_hash():
     try:
-        repo = git.Repo(comfy_path)
-        core.comfy_ui_revision = len(list(repo.iter_commits('HEAD')))
-        core.comfy_ui_commit_datetime = repo.head.commit.committed_datetime
+        from comfyui_manager.common.git_compat import open_repo
+        with open_repo(comfy_path) as repo:
+            core.comfy_ui_revision = repo.iter_commits_count()
+            core.comfy_ui_commit_datetime = repo.head_commit_datetime
     except Exception:
         print('[bold yellow]INFO: Frozen ComfyUI mode.[/bold yellow]')
         core.comfy_ui_revision = 0