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/keys.ts

@@ -0,0 +1,108 @@
+/**
+ * Kitty keyboard protocol key sequence helpers.
+ *
+ * The Kitty keyboard protocol sends enhanced escape sequences in the format:
+ *   \x1b[<codepoint>;<modifier>u
+ *
+ * Modifier values (added to 1):
+ *   - Shift: 1 (value 2)
+ *   - Alt: 2 (value 3)
+ *   - Ctrl: 4 (value 5)
+ *   - Super: 8 (value 9)
+ *
+ * See: https://sw.kovidgoyal.net/kitty/keyboard-protocol/
+ */
+
+// Common codepoints
+const CODEPOINTS = {
+	// Letters (lowercase ASCII)
+	a: 97,
+	c: 99,
+	e: 101,
+	k: 107,
+	o: 111,
+	p: 112,
+	t: 116,
+	u: 117,
+	w: 119,
+
+	// Special keys
+	tab: 9,
+	enter: 13,
+	backspace: 127,
+} as const;
+
+// Modifier bits (before adding 1)
+const MODIFIERS = {
+	shift: 1,
+	alt: 2,
+	ctrl: 4,
+	super: 8,
+} as const;
+
+/**
+ * Build a Kitty keyboard protocol sequence for a key with modifier.
+ */
+function kittySequence(codepoint: number, modifier: number): string {
+	return `\x1b[${codepoint};${modifier + 1}u`;
+}
+
+// Pre-built sequences for common key combinations
+export const Keys = {
+	// Ctrl+<letter> combinations
+	CTRL_A: kittySequence(CODEPOINTS.a, MODIFIERS.ctrl),
+	CTRL_C: kittySequence(CODEPOINTS.c, MODIFIERS.ctrl),
+	CTRL_E: kittySequence(CODEPOINTS.e, MODIFIERS.ctrl),
+	CTRL_K: kittySequence(CODEPOINTS.k, MODIFIERS.ctrl),
+	CTRL_O: kittySequence(CODEPOINTS.o, MODIFIERS.ctrl),
+	CTRL_P: kittySequence(CODEPOINTS.p, MODIFIERS.ctrl),
+	CTRL_T: kittySequence(CODEPOINTS.t, MODIFIERS.ctrl),
+	CTRL_U: kittySequence(CODEPOINTS.u, MODIFIERS.ctrl),
+	CTRL_W: kittySequence(CODEPOINTS.w, MODIFIERS.ctrl),
+
+	// Enter combinations
+	SHIFT_ENTER: kittySequence(CODEPOINTS.enter, MODIFIERS.shift),
+	ALT_ENTER: kittySequence(CODEPOINTS.enter, MODIFIERS.alt),
+	CTRL_ENTER: kittySequence(CODEPOINTS.enter, MODIFIERS.ctrl),
+
+	// Tab combinations
+	SHIFT_TAB: kittySequence(CODEPOINTS.tab, MODIFIERS.shift),
+
+	// Backspace combinations
+	ALT_BACKSPACE: kittySequence(CODEPOINTS.backspace, MODIFIERS.alt),
+} as const;
+
+/**
+ * Check if input matches a Kitty protocol Ctrl+<key> sequence.
+ * @param data - The input data to check
+ * @param key - Single lowercase letter (e.g., 'c' for Ctrl+C)
+ */
+export function isKittyCtrl(data: string, key: string): boolean {
+	if (key.length !== 1) return false;
+	const codepoint = key.charCodeAt(0);
+	return data === kittySequence(codepoint, MODIFIERS.ctrl);
+}
+
+/**
+ * Check if input matches a Kitty protocol key sequence with specific modifier.
+ * @param data - The input data to check
+ * @param codepoint - ASCII codepoint of the key
+ * @param modifier - Modifier value (use MODIFIERS constants)
+ */
+export function isKittyKey(data: string, codepoint: number, modifier: number): boolean {
+	return data === kittySequence(codepoint, modifier);
+}
+
+/**
+ * Check if input matches Ctrl+C (raw byte or Kitty protocol).
+ */
+export function isCtrlC(data: string): boolean {
+	return data === "\x03" || data === Keys.CTRL_C;
+}
+
+/**
+ * Check if input matches Shift+Tab (legacy or Kitty protocol).
+ */
+export function isShiftTab(data: string): boolean {
+	return data === "\x1b[Z" || data === Keys.SHIFT_TAB;
+}