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/docs/hooks.md

@@ -122,8 +122,8 @@ Fired when session changes (`/branch` or session switch).
 
 ```typescript
 pi.on("session_switch", async (event, ctx) => {
-  // event.newSessionFile: string
-  // event.previousSessionFile: string
+  // event.newSessionFile: string | null (null in --no-session mode)
+  // event.previousSessionFile: string | null (null in --no-session mode)
   // event.reason: "branch" | "switch"
 });
 ```