Fix screenshot affecting terminal state in open sessions

- Add get_screen_snapshot() method that doesn't mutate terminal state
- Use change counter for reliable activity detection instead of dirty flag
- Update screenshot handler to use non-mutating snapshot method
- Refactor tests to use shared fixtures and reduce duplication
- Update copilot-instructions.md with detailed Makefile usage

tests/conftest.py

@@ -86,6 +86,7 @@ def mock_session():
     session = MagicMock()
     session.get_screen_has_changes = AsyncMock(return_value=False)
     session.get_screen_state = AsyncMock(return_value=(80, 24, [], True))
+    session.get_screen_snapshot = AsyncMock(return_value=(80, 24, [], True))
     return session
 
 
@@ -95,6 +96,54 @@ def poller() -> Poller:
     return Poller()
 
 
+@pytest.fixture
+def mock_poller() -> MagicMock:
+    """Create a mock Poller for unit tests."""
+    return MagicMock()
+
+
+class DummyAsyncLock:
+    """A dummy async context manager for replacing locks in tests."""
+
+    async def __aenter__(self):
+        return None
+
+    async def __aexit__(self, exc_type, exc, tb):
+        return False
+
+
+@pytest.fixture
+def dummy_lock() -> DummyAsyncLock:
+    """Create a dummy async lock for tests."""
+    return DummyAsyncLock()
+
+
+@pytest.fixture
+def mock_screen_char():
+    """Factory for creating mock pyte screen characters."""
+
+    def _make(
+        data: str = " ",
+        fg: int = 0,
+        bg: int = 0,
+        bold: bool = False,
+        italics: bool = False,
+        underscore: bool = False,
+        reverse: bool = False,
+    ) -> MagicMock:
+        char = MagicMock()
+        char.data = data
+        char.fg = fg
+        char.bg = bg
+        char.bold = bold
+        char.italics = italics
+        char.underscore = underscore
+        char.reverse = reverse
+        return char
+
+    return _make
+
+
 @pytest.fixture
 def session_manager(poller: Poller, tmp_path: Path, sample_terminal_app: App) -> SessionManager:
     """Create a SessionManager instance."""