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/src/entry.tsx

@@ -1,5 +1,4 @@
 #!/usr/bin/env -S node --max-old-space-size=8192 --expose-gc
-import { bootBanner } from './bootBanner.js'
 import { GatewayClient } from './gatewayClient.js'
 import { setupGracefulExit } from './lib/gracefulExit.js'
 import { formatBytes, type HeapDumpResult, performHeapDump } from './lib/memory.js'
@@ -10,8 +9,6 @@ if (!process.stdin.isTTY) {
   process.exit(0)
 }
 
-process.stdout.write(bootBanner())
-
 const gw = new GatewayClient()
 
 gw.start()