feat: persist task lane across submits and session reloads

Task lane questions/actions are now saved to a pending_task_lane JSONB
column on ai_sessions, restoring them on session switch or page reload.
Partial submit no longer force-clears the lane — the AI response
controls what stays. Also removes redundant "New Session" button from
the sidebar (dashboard already provides this).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/ai_session.py

@@ -228,6 +228,7 @@ class AISessionDetail(AISessionSummary):
     ticket_data: dict[str, Any] | None = None
     steps: list[AISessionStepResponse] = []
     conversation_messages: list[dict[str, Any]] = []  # Chat sessions store messages here
+    pending_task_lane: dict[str, Any] | None = None
     is_branching: bool = False
     active_branch_id: str | None = None