TUI: option to configure input cursor style (line/beam instead of block)

Description

The prompt textarea in the TUI always renders a blinking block cursor. There is no way to change it to a thin line (beam), underline, or the terminal default.

Current behavior

• No cursor / cursorStyle option exists in the tui.json schema (checked https://opencode.ai/tui.json) or in opencode.json.
• OpenTUI's EditBufferRenderable defaults to cursorStyle: { style: "block", blinking: true } and calls setCursorStyle on focus, so it overrides the terminal emulator's own cursor setting while the prompt is focused — configuring the terminal does not help.

Requested behavior

A tui.json option, e.g.:

{
  "cursor": { "style": "line", "blinking": true }
}

with values matching OpenTUI's existing CursorStyle type: "block" | "line" | "underline" | "default" (packages/core/src/types.ts in anomalyco/opentui). "default" would leave the terminal's cursor style untouched.

Notes

• The rendering plumbing already exists: EditBufferRenderable accepts a cursorStyle option; opencode's packages/tui/src/component/prompt/index.tsx just never sets it and no config surface exposes it.
• The web app already uses cursorStyle: "bar" for its xterm instances (packages/app/src/components/terminal.tsx), so the TUI is the outlier.