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/load-file.ts

@@ -0,0 +1,35 @@
+/**
+ * Load file into editor - for testing editor scrolling
+ *
+ * Usage: pi --extension ./examples/extensions/load-file.ts
+ *
+ * Commands:
+ *   /load [path]  - Load file into editor (defaults to README.md)
+ */
+
+import * as fs from "node:fs";
+import * as path from "node:path";
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.registerCommand("load", {
+		description: "Load file into editor (defaults to README.md)",
+		handler: async (args, ctx) => {
+			const filePath = args.trim() || "README.md";
+			const fullPath = path.resolve(filePath);
+
+			if (!fs.existsSync(fullPath)) {
+				ctx.ui.notify(`File not found: ${fullPath}`, "error");
+				return;
+			}
+
+			try {
+				const content = fs.readFileSync(fullPath, "utf-8");
+				ctx.ui.setEditorText(content);
+				ctx.ui.notify(`Loaded ${filePath} (${content.split("\n").length} lines)`);
+			} catch (err) {
+				ctx.ui.notify(`Failed to read file: ${err}`, "error");
+			}
+		},
+	});
+}