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/core/hooks/types.ts

@@ -86,10 +86,10 @@ export interface SessionStartEvent {
  */
 export interface SessionSwitchEvent {
 	type: "session_switch";
-	/** New session file path */
-	newSessionFile: string;
-	/** Previous session file path */
-	previousSessionFile: string;
+	/** New session file path, or null in --no-session mode */
+	newSessionFile: string | null;
+	/** Previous session file path, or null in --no-session mode */
+	previousSessionFile: string | null;
 	/** Reason for the switch */
 	reason: "branch" | "switch";
 }