Change branch() to use entryId instead of entryIndex

- AgentSession.branch(entryId: string) now takes entry ID
- SessionBeforeBranchEvent.entryId replaces entryIndex
- getUserMessagesForBranching() returns entryId
- Update RPC types and client
- Update UserMessageSelectorComponent
- Update hook examples and tests
- Update docs (hooks.md, sdk.md)

packages/coding-agent/src/modes/rpc/rpc-mode.ts

@@ -347,7 +347,7 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 			}
 
 			case "branch": {
-				const result = await session.branch(command.entryIndex);
+				const result = await session.branch(command.entryId);
 				return success(id, "branch", { text: result.selectedText, cancelled: result.cancelled });
 			}