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>

sdks/react/package.json

@@ -24,7 +24,7 @@
     "typecheck": "tsc --noEmit"
   },
   "peerDependencies": {
-    "react": "^18.3.1",
+    "react": "^18.3.1 || ^19.0.0",
     "sandbox-agent": "^0.2.2"
   },
   "dependencies": {
@@ -32,8 +32,8 @@
     "ghostty-web": "^0.4.0"
   },
   "devDependencies": {
-    "@types/react": "^18.3.3",
-    "react": "^18.3.1",
+    "@types/react": "^19.1.12",
+    "react": "^19.1.1",
     "sandbox-agent": "workspace:*",
     "tsup": "^8.0.0",
     "typescript": "^5.7.0"