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>

foundry/packages/frontend/src/components/mock-layout/message-list.tsx

@@ -1,8 +1,19 @@
 import { AgentTranscript as AgentTranscript_, type AgentTranscriptClassNames, type TranscriptEntry } from "@sandbox-agent/react";
-
-// Cast to work around React 18/19 type incompatibility between @sandbox-agent/react and foundry
-const AgentTranscript = AgentTranscript_ as unknown as (props: Record<string, unknown>) => JSX.Element;
 import { memo, useEffect, useMemo, type MutableRefObject, type RefObject } from "react";
+
+// Cast needed: tsup-generated .d.ts returns react_jsx_runtime.JSX.Element which
+// doesn't unify with the consumer's JSX.Element under Bundler moduleResolution.
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+const AgentTranscript = AgentTranscript_ as any as React.FC<{
+  entries: TranscriptEntry[];
+  classNames?: Partial<AgentTranscriptClassNames>;
+  scrollRef?: RefObject<HTMLDivElement | null>;
+  scrollToEntryId?: string | null;
+  virtualize?: boolean;
+  isThinking?: boolean;
+  renderMessageText?: (entry: TranscriptEntry) => React.ReactNode;
+  renderThinkingState?: () => React.ReactNode;
+}>;
 import { useStyletron } from "baseui";
 import { LabelSmall, LabelXSmall } from "baseui/typography";
 import { Copy } from "lucide-react";
@@ -156,7 +167,7 @@ export const MessageList = memo(function MessageList({
   pendingMessage,
 }: {
   session: AgentSession | null | undefined;
-  scrollRef: RefObject<HTMLDivElement>;
+  scrollRef: RefObject<HTMLDivElement | null>;
   messageRefs: MutableRefObject<Map<string, HTMLDivElement>>;
   historyEvents: HistoryEvent[];
   onSelectHistoryEvent: (event: HistoryEvent) => void;