The goal evaluation hook was firing on every completed assistant turn
when a goal was active, even for unrelated messages like "what time is
it". This burned the goal budget, triggered continuation prompts that
interrupted unrelated conversations, and made /goal status numbers
misleading.
Add STREAM_GOAL_RELATED and PENDING_GOAL_CONTINUATION flags to gate
the evaluate_goal_after_turn() call in the streaming loop. Only streams
started from goal kickoff (/goal <text>) or goal continuation are
marked as goal-related. Normal user messages skip the hook entirely.
