feat(modal): Add patch approval modal manager

- Create src/patch_modal.py with PatchModalManager class
- Manage patch approval workflow: request, apply, reject
- Provide singleton access via get_patch_modal_manager()
- Add 8 unit tests for modal manager

tests/test_patch_modal.py

@@ -0,0 +1,89 @@
+import pytest
+from src.patch_modal import (
+  PatchModalManager, PendingPatch, 
+  get_patch_modal_manager, reset_patch_modal_manager
+)
+
+def test_patch_modal_manager_init():
+  manager = PatchModalManager()
+  assert manager.get_pending_patch() is None
+  assert manager.is_modal_shown() is False
+
+def test_request_patch_approval():
+  manager = PatchModalManager()
+  patch_text = "--- a/test.py\n+++ b/test.py\n@@ -1 +1 @@\n-old\n+new"
+  file_paths = ["test.py"]
+  
+  result = manager.request_patch_approval(patch_text, file_paths)
+  assert result is True
+  assert manager.is_modal_shown() is True
+  
+  pending = manager.get_pending_patch()
+  assert pending is not None
+  assert pending.patch_text == patch_text
+  assert pending.file_paths == file_paths
+
+def test_reject_patch():
+  manager = PatchModalManager()
+  manager.request_patch_approval("diff", ["file.py"])
+  
+  manager.reject_patch()
+  assert manager.get_pending_patch() is None
+  assert manager.is_modal_shown() is False
+
+def test_close_modal():
+  manager = PatchModalManager()
+  manager.request_patch_approval("diff", ["file.py"])
+  
+  manager.close_modal()
+  assert manager.is_modal_shown() is False
+
+def test_apply_callback():
+  manager = PatchModalManager()
+  called = []
+  
+  def apply_fn(patch: str) -> bool:
+    called.append(patch)
+    return True
+  
+  manager.set_apply_callback(apply_fn)
+  
+  patch_text = "--- a/test.py\n+++ b/test.py\n"
+  result = manager.apply_patch(patch_text)
+  
+  assert result is True
+  assert len(called) == 1
+  assert called[0] == patch_text
+
+def test_reject_callback():
+  manager = PatchModalManager()
+  called = []
+  
+  def reject_fn() -> None:
+    called.append(True)
+  
+  manager.set_reject_callback(reject_fn)
+  manager.reject_patch()
+  
+  assert len(called) == 1
+
+def test_reset():
+  manager = PatchModalManager()
+  manager.request_patch_approval("diff", ["file.py"])
+  manager.set_apply_callback(lambda x: True)
+  manager.set_reject_callback(lambda: None)
+  
+  manager.reset()
+  
+  assert manager.get_pending_patch() is None
+  assert manager.is_modal_shown() is False
+
+def test_get_patch_modal_manager_singleton():
+  reset_patch_modal_manager()
+  
+  mgr1 = get_patch_modal_manager()
+  mgr2 = get_patch_modal_manager()
+  
+  assert mgr1 is mgr2
+  
+  reset_patch_modal_manager()