fix(tasks): prune finished task history

[attid]

Summary

Add automatic retention cleanup for finished background tasks.

Why

Finished task entries (done, failed, cancelled) were retained indefinitely unless a user manually pressed cleanup in /tasks. Since each entry persists the full original_prompt, old finished tasks can make tasks.json grow without bound.

Changes

• Add tasks.finished_retention_hours with a default of 168 hours.
• Add tasks.finished_keep_last with a default of 100 finished tasks.
• Add registry retention cleanup that removes finished entries and their task folders by age and count.
• Keep running and waiting tasks protected from retention cleanup.
• Run task maintenance immediately when the maintenance loop starts, then every 5 hours.

Validation

• uv run pytest tests/tasks tests/test_config.py tests/config -q
• uv run ruff check ductor_bot/config.py ductor_bot/tasks/registry.py ductor_bot/tasks/hub.py tests/tasks/test_registry.py tests/tasks/test_hub.py
• uv run mypy ductor_bot