fix: resolve CI linting issues across codebase

- Remove unused imports (sys, Path, defaultdict, etc.)
- Convert Optional types to modern | syntax
- Fix trailing whitespace and blank line whitespace issues
- Sort imports alphabetically
- Add __future__ annotations for type syntax support

src/codesnap/core/dependency_graph.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+import ast
+from collections import defaultdict
+from pathlib import Path
+from codesnap.core.models import DependencyInfo
+
+
+class DependencyGraph:
+    def __init__(self):
+        self.graph: dict[str, set[str]] = defaultdict(set)
+        self.file_modules: dict[str, str] = {}
+
+    def add_file(self, file_path: Path, tree: ast.AST):
+        module_name = file_path.stem
+        self.file_modules[str(file_path)] = module_name
+
+        for node in ast.walk(tree):
+            if isinstance(node, ast.Import):
+                for alias in node.names:
+                    self.graph[module_name].add(alias.name)
+            elif isinstance(node, ast.ImportFrom):
+                module = node.module or ""
+                self.graph[module_name].add(module)
+
+    def get_dependencies(self, file_path: Path) -> list[DependencyInfo]:
+        module_name = self.file_modules.get(str(file_path), file_path.stem)
+        deps = self.graph.get(module_name, set())
+
+        return [DependencyInfo(name=dep) for dep in deps]
+
+    def visualize(self) -> dict[str, list[str]]:
+        return {k: list(v) for k, v in self.graph.items()}