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/coding-agent/src/modes/interactive/components/user-message-selector.ts

@@ -1,4 +1,4 @@
-import { type Component, Container, Spacer, Text, truncateToWidth } from "@mariozechner/pi-tui";
+import { type Component, Container, Keys, Spacer, Text, truncateToWidth } from "@mariozechner/pi-tui";
 import { theme } from "../theme/theme.js";
 import { DynamicBorder } from "./dynamic-border.js";
 
@@ -99,8 +99,8 @@ class UserMessageList implements Component {
 				this.onCancel();
 			}
 		}
-		// Ctrl+C - cancel
-		else if (keyData === "\x03") {
+		// Ctrl+C - cancel (raw byte or Kitty keyboard protocol)
+		else if (keyData === "\x03" || keyData === Keys.CTRL_C) {
 			if (this.onCancel) {
 				this.onCancel();
 			}