Send a message the user will read. Text outside this tool is visible in the detail view, but most won't open it — the answer lives here. `message` supports markdown. `attachments` takes file paths (absolute or cwd-relative) for images, diffs, logs. `status` labels intent: 'normal' when replying to what they just asked; 'proactive' when you're initiating — a scheduled task finished, a blocker surfaced during background work, you need input on something they haven't asked about. Set it honestly; downstream routing uses it.