fix: add missing json imports and update agent tool tests

Code bugs fixed:
- checkpoint_agent.py, project_agent.py, note_agent.py: add missing
  'import json' (used in handle() for context serialization)

Test fixes:
- test_agents.py: add autouse ws_executor fixture that sets a fake
  execute_on_client so tools can run in unit tests without a WS session
- Rewrite all TestXxxAgentTools tests: patch execute_on_client per-test,
  assert on call_args (what payload was sent to the client) and on the
  formatted string return value — matching actual tool behavior

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/agents/checkpoint_agent.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import json
 from typing import Any
 
 from langchain_core.messages import HumanMessage, SystemMessage