Export tasks as CSV

Export a todo-mode board's columns + tasks as CSV — one row per task, RFC-4180 quoted. Header: lane,column,name,priority,assignee,due_date,done,description. lane is the title of the swimlane (row) the task's column sits in, falling back to the lane's 1-based row number (1, 2, …) when that lane has no title. Mirrors the MCP export_tasks tool with format: "csv". A draw board (no columns) still returns 200 with just the header row, so callers don't have to branch on mode first.