feat(agents): implement agent framework and CLI

tests/fixtures/simple.diff

@@ -0,0 +1,16 @@
+diff --git a/src/utils.py b/src/utils.py
+index 1234567..abcdefg 100644
+--- a/src/utils.py
++++ b/src/utils.py
+@@ -1,5 +1,8 @@
+ """Utility functions."""
+
+
++def add(a: int, b: int) -> int:
++    """Add two numbers."""
++    return a + b
++
++
+ def subtract(a: int, b: int) -> int:
+     """Subtract two numbers."""
+     return a - b