fix(coding-agent): fork writes to new session file, not parent (fixes #1242)

- Store previousSessionFile before creating branched session
- Update sessionFile after writing new branch file
- Pass parentSession when forking from first message
- Add --local to git config in tests to prevent repo escape

packages/coding-agent/src/core/agent-session.ts

@@ -2370,7 +2370,7 @@ export class AgentSession {
 		this._pendingNextTurnMessages = [];
 
 		if (!selectedEntry.parentId) {
-			this.sessionManager.newSession();
+			this.sessionManager.newSession({ parentSession: previousSessionFile });
 		} else {
 			this.sessionManager.createBranchedSession(selectedEntry.parentId);
 		}