docs: improve Git.init documentation and validation tests

- Enhance Git.init docstrings with detailed parameter descriptions
- Add comprehensive examples including SHA-256 object format
- Add return value and exception documentation
- Improve type hints for shared parameter with Literal types
- Add extensive validation tests for all parameters

Author: tony

src/libvcs/cmd/git.py

@@ -1035,7 +1035,10 @@ def init(
         object_format: t.Literal["sha1", "sha256"] | None = None,
         branch: str | None = None,
         initial_branch: str | None = None,
-        shared: bool | str | None = None,
+        shared: bool
+        | Literal[false, true, umask, group, all, world, everybody]
+        | str
+        | None = None,
         quiet: bool | None = None,
         bare: bool | None = None,
         # libvcs special behavior
@@ -1049,28 +1052,58 @@ def init(
         template : str, optional
             Directory from which templates will be used. The template directory
             contains files and directories that will be copied to the $GIT_DIR
-            after it is created.
+            after it is created. The template directory will be one of the
+            following (in order):
+            - The argument given with the --template option
+            - The contents of the $GIT_TEMPLATE_DIR environment variable
+            - The init.templateDir configuration variable
+            - The default template directory: /usr/share/git-core/templates
         separate_git_dir : :attr:`libvcs._internal.types.StrOrBytesPath`, optional
             Instead of placing the git repository in <directory>/.git/, place it in
-            the specified path.
+            the specified path. The .git file at <directory>/.git will contain a
+            gitfile that points to the separate git dir. This is useful when you
+            want to store the git directory on a different disk or filesystem.
         object_format : "sha1" | "sha256", optional
             Specify the hash algorithm to use. The default is sha1. Note that
-            sha256 is still experimental in git.
+            sha256 is still experimental in git and requires git version >= 2.29.0.
+            Once the repository is created with a specific hash algorithm, it cannot
+            be changed.
         branch : str, optional
             Use the specified name for the initial branch. If not specified, fall
-            back to the default name (currently "master").
+            back to the default name (currently "master", but may change based on
+            init.defaultBranch configuration).
         initial_branch : str, optional
             Alias for branch parameter. Specify the name for the initial branch.
+            This is provided for compatibility with newer git versions.
         shared : bool | str, optional
             Specify that the git repository is to be shared amongst several users.
-            Can be 'false', 'true', 'umask', 'group', 'all', 'world',
-            'everybody', or an octal number.
+            Valid values are:
+            - false: Turn off sharing (default)
+            - true: Same as group
+            - umask: Use permissions specified by umask
+            - group: Make the repository group-writable
+            - all, world, everybody: Same as world, make repo readable by all users
+            - An octal number: Explicit mode specification (e.g., "0660")
         quiet : bool, optional
             Only print error and warning messages; all other output will be
-            suppressed.
+            suppressed. Useful for scripting.
         bare : bool, optional
             Create a bare repository. If GIT_DIR environment is not set, it is set
-            to the current working directory.
+            to the current working directory. Bare repositories have no working
+            tree and are typically used as central repositories.
+        check_returncode : bool, optional
+            If True, check the return code of the git command and raise a
+            CalledProcessError if it is non-zero.
+
+        Returns
+        -------
+        str
+            The output of the git init command.
+
+        Raises
+        ------
+        CalledProcessError
+            If the git command fails and check_returncode is True.
 
         Examples
         --------
@@ -1119,6 +1152,13 @@ def init(
         >>> git = Git(path=template_repo)
         >>> git.init(template=str(tmp_path))
         'Initialized empty Git repository in ...'
+
+        Create with SHA-256 object format (requires git >= 2.29.0):
+
+        >>> sha256_repo = tmp_path / 'sha256_example'
+        >>> sha256_repo.mkdir()
+        >>> git = Git(path=sha256_repo)
+        >>> git.init(object_format='sha256')  # doctest: +SKIP
         """
         local_flags: list[str] = []
         required_flags: list[str] = [str(self.path)]

tests/cmd/test_git.py

@@ -132,3 +132,42 @@ def test_git_reinit(tmp_path: pathlib.Path) -> None:
     # Reinit
     second_result = repo.init()
     assert "Reinitialized existing Git repository" in second_result
+
+
+def test_git_init_validation_errors(tmp_path: pathlib.Path) -> None:
+    """Test validation errors in git init."""
+    repo = git.Git(path=tmp_path)
+
+    # Test invalid template type
+    with pytest.raises(TypeError, match="template must be a string or Path"):
+        repo.init(template=123)  # type: ignore
+
+    # Test non-existent template directory
+    with pytest.raises(ValueError, match="template directory does not exist"):
+        repo.init(template=str(tmp_path / "nonexistent"))
+
+    # Test invalid object format
+    with pytest.raises(
+        ValueError,
+        match="object_format must be either 'sha1' or 'sha256'",
+    ):
+        repo.init(object_format="invalid")  # type: ignore
+
+    # Test specifying both branch and initial_branch
+    with pytest.raises(
+        ValueError,
+        match="Cannot specify both branch and initial_branch",
+    ):
+        repo.init(branch="main", initial_branch="master")
+
+    # Test branch name with whitespace
+    with pytest.raises(ValueError, match="Branch name cannot contain whitespace"):
+        repo.init(branch="main branch")
+
+    # Test invalid shared value
+    with pytest.raises(ValueError, match="Invalid shared value"):
+        repo.init(shared="invalid")
+
+    # Test invalid octal number for shared
+    with pytest.raises(ValueError, match="Invalid shared value"):
+        repo.init(shared="8888")  # Invalid octal number