Write unit tests for planning system

[xmaks82]

Description

The planning system (src/agent/planning/) has no unit tests.

Scope

• tests/test_task.py — Task dataclass, create/complete/start/block/serialization
• tests/test_task_manager.py — TaskManager CRUD, list, filter, clear_completed

Notes

• Use pytest and pytest-asyncio
• Use temporary file for JSONL storage in tests