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

[PleasePrompto]

Thanks — the core logic here is solid: the retention pruning can't touch running/waiting tasks, the age+count criteria merge cleanly, and the hub maintenance test is well built. A few things before un-drafting:

1. Consolidate cleanup_old (tasks/registry.py): it has no production caller (only its own tests) and overlaps with the new cleanup_finished_retention. Please either remove it (incl. TestCleanupOld) or fold the new logic into it — two parallel age-pruning methods is exactly the kind of dead code this repo's conventions forbid.
2. config.example.json: add finished_retention_hours / finished_keep_last to the tasks block (the other fields are all listed there).
3. Docs: docs/config.md (TasksConfig table) and docs/modules/tasks.md need the new fields; the "orphan cleanup runs every 5 hours" note should also mention the maintenance pass now runs immediately at startup.
4. Optional but nice: a test for the age×count interaction (young task evicted by keep_last) and the <=0 noop path.

Happy to merge once those are in. 👍