Expose create_space over MCP (unblocks onboarding seed auto-structuring)

You can now create a Shared Space directly from your AI assistant (via MCP), so onboarding can set up your team's Spaces without leaving the chat. The new create_space tool handles org resolution automatically — if you belong to exactly one organization it picks it for you, and it gracefully blocks creation when there's no active subscription. Calling create_space with the same name twice is safe: if the Space already exists, the existing one is returned unchanged.