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/terminal.ts

@@ -51,6 +51,12 @@ export class ProcessTerminal implements Terminal {
 		// Enable bracketed paste mode - terminal will wrap pastes in \x1b[200~ ... \x1b[201~
 		process.stdout.write("\x1b[?2004h");
 
+		// Enable Kitty keyboard protocol (disambiguate escape codes)
+		// This makes terminals like Ghostty, Kitty, WezTerm send enhanced key sequences
+		// e.g., Shift+Enter becomes \x1b[13;2u instead of just \r
+		// See: https://sw.kovidgoyal.net/kitty/keyboard-protocol/
+		process.stdout.write("\x1b[>1u");
+
 		// Set up event handlers
 		process.stdin.on("data", this.inputHandler);
 		process.stdout.on("resize", this.resizeHandler);
@@ -60,6 +66,9 @@ export class ProcessTerminal implements Terminal {
 		// Disable bracketed paste mode
 		process.stdout.write("\x1b[?2004l");
 
+		// Disable Kitty keyboard protocol (pop the flags we pushed)
+		process.stdout.write("\x1b[<u");
+
 		// Remove event handlers
 		if (this.inputHandler) {
 			process.stdin.removeListener("data", this.inputHandler);