feat(context): Implement directory grouping and file stats in context composition panel

tests/test_context_composition_phase3.py

@@ -0,0 +1,30 @@
+import pytest
+from src.aggregate import group_files_by_dir, compute_file_stats
+from src.models import FileItem
+
+def test_group_files_by_dir():
+    files = [
+        FileItem(path="src/main.py"),
+        FileItem(path="src/utils/helpers.py"),
+        FileItem(path="tests/test_main.py"),
+        FileItem(path="README.md")
+    ]
+    grouped = group_files_by_dir(files)
+    assert len(grouped) == 4
+    assert grouped["src"] == [files[0]]
+    assert grouped["src/utils"] == [files[1]]
+    assert grouped["tests"] == [files[2]]
+    assert grouped["."] == [files[3]]
+
+def test_compute_file_stats():
+    import tempfile
+    import os
+    with tempfile.TemporaryDirectory() as temp_dir:
+        # Create a dummy python file
+        py_path = os.path.join(temp_dir, "test.py")
+        with open(py_path, "w") as f:
+            f.write("def foo():\n    pass\n\nclass Bar:\n    pass\n")
+        
+        stats = compute_file_stats(py_path)
+        assert stats["lines"] == 5
+        assert stats["ast_elements"] == 2 # 1 func, 1 class