fix(tui): add vertical scrolling to Editor when content exceeds terminal height

The Editor component now accepts TUI as the first constructor parameter,
enabling it to query terminal dimensions. When content exceeds available
height, the editor scrolls vertically keeping the cursor visible.

Features:
- Max editor height is 30% of terminal rows (minimum 5 lines)
- Page Up/Down keys scroll by page size
- Scroll indicators show lines above/below: ─── ↑ 5 more ───

Breaking change: Editor constructor signature changed from
  new Editor(theme)
to
  new Editor(tui, theme)

fixes #732

packages/coding-agent/examples/extensions/modal-editor.ts

@@ -80,6 +80,6 @@ class ModalEditor extends CustomEditor {
 
 export default function (pi: ExtensionAPI) {
 	pi.on("session_start", (_event, ctx) => {
-		ctx.ui.setEditorComponent((_tui, theme, kb) => new ModalEditor(theme, kb));
+		ctx.ui.setEditorComponent((tui, theme, kb) => new ModalEditor(tui, theme, kb));
 	});
 }