Recognize Esc and other control keys in kitty keyboard mode

Enabling the kitty keyboard protocol for Shift+Enter made terminals
report Esc as CSI 27 u, which the CSI-u parser dropped as KeyUnknown,
so Esc stopped aborting the agent. Map kitty control codepoints
(Esc=27, Tab=9, Backspace=127/8) back to their dedicated keys.

packages/tui/input.go

@@ -342,9 +342,22 @@ func keyFromModifiedCode(code, mod int) (Key, bool) {
 	shift := bits&1 != 0
 	alt := bits&2 != 0
 	ctrl := bits&4 != 0
+	// Kitty keyboard protocol (CSI ... u) reports control keys as their
+	// codepoints: Esc=27, Enter=13, Tab=9, Backspace=127. Without the
+	// enhanced-mode handling these arrive as raw bytes; with it enabled
+	// they come through here, so map them back to their dedicated keys.
 	switch code {
 	case 13:
 		return Key{Kind: KeyEnter, Shift: shift, Alt: alt, Ctrl: ctrl}, true
+	case 27:
+		return Key{Kind: KeyEsc, Shift: shift, Alt: alt, Ctrl: ctrl}, true
+	case 9:
+		if shift {
+			return Key{Kind: KeyShiftTab, Alt: alt, Ctrl: ctrl}, true
+		}
+		return Key{Kind: KeyTab, Shift: shift, Alt: alt, Ctrl: ctrl}, true
+	case 127, 8:
+		return Key{Kind: KeyBackspace, Shift: shift, Alt: alt, Ctrl: ctrl}, true
 	}
 	if ctrl {
 		switch code {

packages/tui/input_test.go

@@ -30,6 +30,25 @@ func TestReaderParsesModifyOtherKeysCtrlC(t *testing.T) {
 	}
 }
 
+func TestReaderParsesCSIUEsc(t *testing.T) {
+	k := readKey(t, "\x1b[27u")
+	if k.Kind != KeyEsc {
+		t.Fatalf("Read kind=%v, want esc", k.Kind)
+	}
+}
+
+func TestReaderParsesCSIUTabAndBackspace(t *testing.T) {
+	if k := readKey(t, "\x1b[9u"); k.Kind != KeyTab {
+		t.Fatalf("Read kind=%v, want tab", k.Kind)
+	}
+	if k := readKey(t, "\x1b[9;2u"); k.Kind != KeyShiftTab {
+		t.Fatalf("Read kind=%v, want shift-tab", k.Kind)
+	}
+	if k := readKey(t, "\x1b[127u"); k.Kind != KeyBackspace {
+		t.Fatalf("Read kind=%v, want backspace", k.Kind)
+	}
+}
+
 func TestReaderParsesSGRMouseWheel(t *testing.T) {
 	cases := []struct {
 		seq  string