feat: implement task dependencies with validation and update handling

backend/tests/test_board_schema.py

@@ -1,6 +1,7 @@
-import pytest
 from uuid import uuid4
 
+import pytest
+
 from app.schemas.board_onboarding import BoardOnboardingConfirm
 from app.schemas.boards import BoardCreate
 

backend/tests/test_mentions.py

@@ -17,4 +17,3 @@ def test_matches_agent_mention_supports_reserved_lead_shortcut():
     other = Agent(name="Lead", is_board_lead=False)
     assert matches_agent_mention(lead, {"lead"}) is True
     assert matches_agent_mention(other, {"lead"}) is False
-