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/interactive/interactive-mode.ts

@@ -1570,9 +1570,9 @@ export class InteractiveMode {
 
 		this.showSelector((done) => {
 			const selector = new UserMessageSelectorComponent(
-				userMessages.map((m) => ({ index: m.entryIndex, text: m.text })),
-				async (entryIndex) => {
-					const result = await this.session.branch(entryIndex);
+				userMessages.map((m) => ({ id: m.entryId, text: m.text })),
+				async (entryId) => {
+					const result = await this.session.branch(entryId);
 					if (result.cancelled) {
 						// Hook cancelled the branch
 						done();