Fix branch selector for single message and --no-session mode

- Allow branch selector to open with single user message (changed <= 1 to === 0 check)
- Support in-memory branching for --no-session mode (no files created)
- Add isEnabled() getter to SessionManager
- Update sessionFile getter to return null when sessions disabled
- Update SessionSwitchEvent types to allow null session files
- Add branching tests for single message and --no-session scenarios

fixes #163

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

@@ -136,8 +136,8 @@ export class UserMessageSelectorComponent extends Container {
 		this.addChild(new Spacer(1));
 		this.addChild(new DynamicBorder());
 
-		// Auto-cancel if no messages or only one message
-		if (messages.length <= 1) {
+		// Auto-cancel if no messages
+		if (messages.length === 0) {
 			setTimeout(() => onCancel(), 100);
 		}
 	}