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>

CLAUDE.md

@@ -93,15 +93,21 @@ When adding a new agent, update all of the following:
   - `docs/sdk-overview.mdx`
   - `docs/react-components.mdx`
   - `docs/session-persistence.mdx`
+  - `docs/architecture.mdx`
   - `docs/deploy/local.mdx`
   - `docs/deploy/cloudflare.mdx`
   - `docs/deploy/vercel.mdx`
   - `docs/deploy/daytona.mdx`
   - `docs/deploy/e2b.mdx`
   - `docs/deploy/docker.mdx`
+  - `docs/deploy/boxlite.mdx`
+  - `docs/deploy/modal.mdx`
+  - `docs/deploy/computesdk.mdx`
   - `frontend/packages/website/src/components/GetStarted.tsx`
   - `.claude/commands/post-release-testing.md`
   - `examples/cloudflare/Dockerfile`
+  - `examples/boxlite/Dockerfile`
+  - `examples/boxlite-python/Dockerfile`
   - `examples/daytona/src/index.ts`
   - `examples/shared/src/docker.ts`
   - `examples/docker/src/index.ts`