Rename /branch command to /fork

- RPC: branch -> fork, get_branch_messages -> get_fork_messages
- SDK: branch() -> fork(), getBranchMessages() -> getForkMessages()
- AgentSession: branch() -> fork(), getUserMessagesForBranching() -> getUserMessagesForForking()
- Extension events: session_before_branch -> session_before_fork, session_branch -> session_fork
- Settings: doubleEscapeAction 'branch' -> 'fork'

fixes #641

packages/coding-agent/src/index.ts

@@ -67,12 +67,12 @@ export type {
 	MessageRenderOptions,
 	RegisteredCommand,
 	RegisteredTool,
-	SessionBeforeBranchEvent,
 	SessionBeforeCompactEvent,
+	SessionBeforeForkEvent,
 	SessionBeforeSwitchEvent,
 	SessionBeforeTreeEvent,
-	SessionBranchEvent,
 	SessionCompactEvent,
+	SessionForkEvent,
 	SessionShutdownEvent,
 	SessionStartEvent,
 	SessionSwitchEvent,