fix: preserve task board interventions

[zhanghanduo]

Summary

• mark TaskBoardObserver critical so notify_observers awaits it and collects its Intervention
• add dispatcher-level regression coverage for the observer contract
• add a real run_agent_loop end-to-end test proving the task-board reminder reaches the next LLM request
• audit production observers for other non-critical hooks returning control values; no additional cases found

Root cause

Non-critical observer hooks are dispatched as fire-and-forget tasks and their return values are intentionally ignored. TaskBoardObserver returned Intervention(inject_messages=...) while declaring critical=False, so the reminder was silently dropped.

Validation

• uv run --extra dev ruff check frontier_agent/components/observers/task_board.py tests/test_task_board_intervention.py
• uv run --extra dev pytest -q
• 1706 passed, 4 skipped