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/rpc/rpc-types.ts

@@ -65,7 +65,7 @@ export interface RpcSessionState {
 	isStreaming: boolean;
 	isCompacting: boolean;
 	queueMode: "all" | "one-at-a-time";
-	sessionFile: string;
+	sessionFile: string | null;
 	sessionId: string;
 	autoCompactionEnabled: boolean;
 	messageCount: number;