feat: extract TaskStore into pluggable store abstraction layer

[he-yufeng]

Summary

• Mirrors the existing transport/ pattern (Transport ABC → FileTransport → factory) for task storage
• Extracts BaseTaskStore ABC into clawteam/store/base.py with a default get_stats() implementation
• Moves the file-based implementation into FileTaskStore at clawteam/store/file.py (behavior unchanged)
• Adds get_task_store() factory that checks CLAWTEAM_TASK_STORE env var / config before falling back to file
• Adds task_store field to ClawTeamConfig + env var mapping in get_effective()
• Existing imports (from clawteam.team.tasks import TaskStore) keep working via lazy re-export

This is Phase 1 from the roadmap — it unblocks adding Redis/SQL backends later without touching any existing callers.

New layout

clawteam/store/
├── __init__.py   # get_task_store() factory + re-exports
├── base.py       # BaseTaskStore ABC
└── file.py       # FileTaskStore (extracted from team/tasks.py)

Test plan

• 17 new tests covering ABC, factory, backward compat, config integration, round-trip CRUD
• All 201 tests passing (184 existing + 17 new)
• Ruff clean