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>

server/packages/agent-management/src/agents.rs

@@ -1090,9 +1090,9 @@ fn write_mock_agent_process_launcher(path: &Path) -> Result<(), AgentError> {
         fs::create_dir_all(parent)?;
     }
     let script = if cfg!(windows) {
-        "@echo off\r\necho mock agent process is in-process in sandbox-agent\r\nexit /b 1\r\n"
+        "@echo off\r\nsandbox-agent mock-agent-process %*\r\n"
     } else {
-        "#!/usr/bin/env sh\necho 'mock agent process is in-process in sandbox-agent'\nexit 1\n"
+        "#!/usr/bin/env sh\nexec sandbox-agent mock-agent-process \"$@\"\n"
     };
     write_text_file(path, script)
 }