fix: mock agent process, React 18/19 types, release version refs

- Add hidden `mock-agent-process` CLI subcommand implementing a stdio
  JSON-RPC echo agent (ported from examples/mock-acp-agent)
- Update write_mock_agent_process_launcher() to exec the new subcommand
  instead of exiting with error
- Update sdks/react to support React 18 and 19 peer dependencies
- Update @types/react to v19 across workspace (pnpm override + inspector)
- Fix RefObject<T | null> compatibility for React 19 useRef() signatures
- Add version reference replacement logic to release update_version.ts
  covering all docs, examples, and code files listed in CLAUDE.md
- Add missing files to CLAUDE.md Install Version References list
  (architecture.mdx, boxlite, modal, computesdk docs and examples)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/packages/inspector/src/components/chat/ChatPanel.tsx

@@ -71,7 +71,7 @@ const ChatPanel = ({
   agents: AgentInfo[];
   agentsLoading: boolean;
   agentsError: string | null;
-  scrollRef: RefObject<HTMLDivElement>;
+  scrollRef: RefObject<HTMLDivElement | null>;
   agentLabel: string;
   modelLabel?: string | null;
   currentAgentVersion?: string | null;

frontend/packages/inspector/src/components/chat/InspectorConversation.tsx

@@ -84,7 +84,7 @@ export interface InspectorConversationProps {
   entries: TranscriptEntry[];
   sessionError: string | null;
   eventError?: string | null;
-  scrollRef: RefObject<HTMLDivElement>;
+  scrollRef: RefObject<HTMLDivElement | null>;
   onEventClick?: (eventId: string) => void;
   isThinking?: boolean;
   agentId?: string;