Add Kitty keyboard protocol support for Shift+Enter and other modifier keys

Enable the Kitty keyboard protocol on terminal start to receive enhanced
key sequences that include modifier information. This fixes Shift+Enter
not working in Ghostty, Kitty, WezTerm, and other modern terminals.

Changes:
- Enable Kitty protocol on start (\x1b[>1u), disable on stop (\x1b[<u)
- Add centralized key definitions in packages/tui/src/keys.ts
- Support both legacy and Kitty sequences for all modifier+key combos:
  - Shift+Enter, Alt+Enter for newlines
  - Shift+Tab for thinking level cycling
  - Ctrl+C, Ctrl+A, Ctrl+E, Ctrl+K, Ctrl+U, Ctrl+W, Ctrl+O, Ctrl+P, Ctrl+T
  - Alt+Backspace for word deletion
- Export Keys constants and helper functions from @mariozechner/pi-tui

packages/tui/src/index.ts

@@ -18,6 +18,8 @@ export { type SelectItem, SelectList, type SelectListTheme } from "./components/
 export { Spacer } from "./components/spacer.js";
 export { Text } from "./components/text.js";
 export { TruncatedText } from "./components/truncated-text.js";
+// Kitty keyboard protocol helpers
+export { isCtrlC, isKittyCtrl, isKittyKey, isShiftTab, Keys } from "./keys.js";
 // Terminal interface and implementations
 export { ProcessTerminal, type Terminal } from "./terminal.js";
 // Terminal image support