fix(tui): drop main-screen banner flash, widen alt-screen clear on entry

- entry.tsx no longer writes bootBanner() to the main screen before the
  alt-screen enters. The <Banner> renders inside the alt screen via the
  seeded intro row, so nothing is lost — just the flash that preceded it.
  Fixes the torn first frame reported on Alacritty (blitz row 5 #17) and
  shaves the 'starting agent' hang perception (row 5 #1) since the UI
  paints straight into the steady-state view
- AlternateScreen prefixes ERASE_SCROLLBACK (\x1b[3J) to its entry so
  strict emulators start from a pristine grid; named constants replace
  the inline sequences for clarity
- bootBanner.ts deleted — dead code

ui-tui/packages/hermes-ink/src/ink/components/AlternateScreen.tsx

@@ -2,6 +2,7 @@ import React, { type PropsWithChildren, useContext, useInsertionEffect } from 'r
 import { c as _c } from 'react/compiler-runtime'
 
 import instances from '../instances.js'
+import { CURSOR_HOME, ERASE_SCREEN, ERASE_SCROLLBACK } from '../termio/csi.js'
 import { DISABLE_MOUSE_TRACKING, ENABLE_MOUSE_TRACKING, ENTER_ALT_SCREEN, EXIT_ALT_SCREEN } from '../termio/dec.js'
 import { TerminalWriteContext } from '../useTerminalNotification.js'
 
@@ -51,7 +52,9 @@ export function AlternateScreen(t0: Props) {
         return
       }
 
-      writeRaw(ENTER_ALT_SCREEN + '\x1B[2J\x1B[H' + (mouseTracking ? ENABLE_MOUSE_TRACKING : ''))
+      writeRaw(
+        ENTER_ALT_SCREEN + ERASE_SCROLLBACK + ERASE_SCREEN + CURSOR_HOME + (mouseTracking ? ENABLE_MOUSE_TRACKING : '')
+      )
       ink?.setAltScreenActive(true, mouseTracking)
 
       return () => {