exc(refactor) Remove unused worktree exception classes

why: WorktreeExistsError, WorktreeRefNotFoundError, and WorktreeDirtyError
are never imported, raised, or caught anywhere in the codebase. The worktree
sync code uses WorktreeSyncResult entries with error strings instead.
what:
- Remove WorktreeExistsError class
- Remove WorktreeRefNotFoundError class
- Remove WorktreeDirtyError class
- Keep WorktreeError (base) and WorktreeConfigError (actively used)

Author: tony

src/vcspull/exc.py

@@ -17,43 +17,8 @@ class WorktreeError(VCSPullException):
     """Base exception for worktree operations."""
 
 
-class WorktreeExistsError(WorktreeError):
-    """Worktree already exists at the specified path."""
-
-    def __init__(self, path: str, *args: object, **kwargs: object) -> None:
-        super().__init__(f"Worktree already exists at path: {path}")
-        self.path = path
-
-
-class WorktreeRefNotFoundError(WorktreeError):
-    """Reference (tag, branch, or commit) not found in repository."""
-
-    def __init__(
-        self,
-        ref: str,
-        ref_type: str,
-        repo_path: str,
-        *args: object,
-        **kwargs: object,
-    ) -> None:
-        super().__init__(
-            f"{ref_type.capitalize()} '{ref}' not found in repository at {repo_path}"
-        )
-        self.ref = ref
-        self.ref_type = ref_type
-        self.repo_path = repo_path
-
-
 class WorktreeConfigError(WorktreeError):
     """Invalid worktree configuration."""
 
     def __init__(self, message: str, *args: object, **kwargs: object) -> None:
         super().__init__(message)
-
-
-class WorktreeDirtyError(WorktreeError):
-    """Worktree has uncommitted changes and cannot be modified."""
-
-    def __init__(self, path: str, *args: object, **kwargs: object) -> None:
-        super().__init__(f"Worktree at {path} has uncommitted changes")
-        self.path = path