fix(tui): reduce unnecessary full redraws for better performance

- Remove height change detection (only width changes trigger full redraw)
- Change clearOnShrink default to false (use PI_CLEAR_ON_SHRINK=1 to enable)
- Fix viewport check to use previousLines.length instead of maxLinesRendered
  (prevents false positive redraws when appending lines after content shrunk)
- Add clearOnShrink setting to /settings in coding-agent
- Remove line truncation in custom message component (always show full content)

packages/coding-agent/src/modes/interactive/components/custom-message.ts

@@ -90,14 +90,6 @@ export class CustomMessageComponent extends Container {
 				.join("\n");
 		}
 
-		// Limit lines when collapsed
-		if (!this._expanded) {
-			const lines = text.split("\n");
-			if (lines.length > 5) {
-				text = `${lines.slice(0, 5).join("\n")}\n...`;
-			}
-		}
-
 		this.box.addChild(
 			new Markdown(text, 0, 0, this.markdownTheme, {
 				color: (text: string) => theme.fg("customMessageText", text),