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/examples/extensions/git-checkpoint.ts

@@ -1,8 +1,8 @@
 /**
  * Git Checkpoint Extension
  *
- * Creates git stash checkpoints at each turn so /branch can restore code state.
- * When branching, offers to restore code to that point in history.
+ * Creates git stash checkpoints at each turn so /fork can restore code state.
+ * When forking, offers to restore code to that point in history.
  */
 
 import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
@@ -26,7 +26,7 @@ export default function (pi: ExtensionAPI) {
 		}
 	});
 
-	pi.on("session_before_branch", async (event, ctx) => {
+	pi.on("session_before_fork", async (event, ctx) => {
 		const ref = checkpoints.get(event.entryId);
 		if (!ref) return;