fix(coordination): remove spurious type-ignore on AgentBudgetTracker.subscribe_changes #8708

Problem

autobot-backend/llc/services/agent_budget_tracker.py:

async def subscribe_changes(self):  # type: ignore[override]

AgentBudgetTracker has no base class defining subscribe_changes, so # type: ignore[override] suppresses a non-existent error. Remove it.

Discovery

Found during CodeReviewer review of PR #8693 / GH#6630.