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/components/select-list.ts

@@ -1,3 +1,4 @@
+import { Keys } from "../keys.js";
 import type { Component } from "../tui.js";
 import { truncateToWidth } from "../utils.js";
 
@@ -161,8 +162,8 @@ export class SelectList implements Component {
 				this.onSelect(selectedItem);
 			}
 		}
-		// Escape or Ctrl+C
-		else if (keyData === "\x1b" || keyData === "\x03") {
+		// Escape or Ctrl+C (raw byte or Kitty keyboard protocol)
+		else if (keyData === "\x1b" || keyData === "\x03" || keyData === Keys.CTRL_C) {
 			if (this.onCancel) {
 				this.onCancel();
 			}