fix(tui): stabilize sticky prompt tracking

Keep the latest prompt sticky while the viewport is in live assistant output beyond history, and clear stale sticky state at the real bottom using fresh scroll height.

ui-tui/src/types/hermes-ink.d.ts

@@ -83,6 +83,7 @@ declare module '@hermes/ink' {
     readonly getScrollTop: () => number
     readonly getPendingDelta: () => number
     readonly getScrollHeight: () => number
+    readonly getFreshScrollHeight: () => number
     readonly getViewportHeight: () => number
     readonly getViewportTop: () => number
     readonly getLastManualScrollAt: () => number