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/interactive-mode.ts

@@ -1214,7 +1214,7 @@ export class InteractiveMode {
 	private showUserMessageSelector(): void {
 		const userMessages = this.session.getUserMessagesForBranching();
 
-		if (userMessages.length <= 1) {
+		if (userMessages.length === 0) {
 			this.showStatus("No messages to branch from");
 			return;
 		}