Fix sdk.md and rpc.md to match actual API

- Remove incorrect prompt(AppMessage) overload
- Change AppMessage to AgentMessage
- Change null to undefined for optional returns
- sendHookMessage returns Promise<void>
- Update rpc.md for entryId change

packages/coding-agent/docs/rpc.md

@@ -491,7 +491,7 @@ If a hook cancelled the switch:
 Create a new branch from a previous user message. Can be cancelled by a `before_branch` hook. Returns the text of the message being branched from.
 
 ```json
-{"type": "branch", "entryIndex": 2}
+{"type": "branch", "entryId": "abc123"}
 ```
 
 Response:
@@ -530,8 +530,8 @@ Response:
   "success": true,
   "data": {
     "messages": [
-      {"entryIndex": 0, "text": "First prompt..."},
-      {"entryIndex": 2, "text": "Second prompt..."}
+      {"entryId": "abc123", "text": "First prompt..."},
+      {"entryId": "def456", "text": "Second prompt..."}
     ]
   }
 }