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/core/index.ts

@@ -41,12 +41,12 @@ export {
 	type LoadExtensionsResult,
 	type MessageRenderer,
 	type RegisteredCommand,
-	type SessionBeforeBranchEvent,
 	type SessionBeforeCompactEvent,
+	type SessionBeforeForkEvent,
 	type SessionBeforeSwitchEvent,
 	type SessionBeforeTreeEvent,
-	type SessionBranchEvent,
 	type SessionCompactEvent,
+	type SessionForkEvent,
 	type SessionShutdownEvent,
 	type SessionStartEvent,
 	type SessionSwitchEvent,