Add helper functions for key detection and update usage

- Add isCtrlA/C/E/K/O/P/T/U/W helper functions that check both raw and Kitty formats
- Add isAltBackspace helper function
- Refactor editor.ts, input.ts, select-list.ts to use helper functions
- Refactor custom-editor.ts, session-selector.ts, user-message-selector.ts
- Add CHANGELOG entry for the Shift+Enter fix

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

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