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/docs/sdk.md

@@ -108,8 +108,8 @@ interface AgentSession {
   newSession(options?: { parentSession?: string }): Promise<boolean>;  // Returns false if cancelled by hook
   switchSession(sessionPath: string): Promise<boolean>;
   
-  // Branching
-  branch(entryId: string): Promise<{ selectedText: string; cancelled: boolean }>;  // Creates new session file
+  // Forking
+  fork(entryId: string): Promise<{ selectedText: string; cancelled: boolean }>;  // Creates new session file
   navigateTree(targetId: string, options?: { summarize?: boolean }): Promise<{ editorText?: string; cancelled: boolean }>;  // In-place navigation
   
   // Hook message injection