[codex] Add Node.js web sandbox example

[GodBlf]

Summary

• Add examples/node-web-sandbox, a Node.js 20 web-service sandbox template/example with Dockerfile, startup script, smoke test, E2B-compatible validation script, and English/Chinese README files.
• Register the example in the English and Chinese tutorial example indexes.
• Add spec-kit artifacts for issue 645, including spec, plan, tasks, quickstart, contracts, and checklist.

Refs #645
Issue: #645

Validation

• node --check examples/node-web-sandbox/server.js
• python3 -m py_compile examples/node-web-sandbox/smoke_test.py examples/node-web-sandbox/validate.py
• Secret scan over examples/node-web-sandbox and the updated docs indexes.
• CubeSandbox test deployment validation on Linux x86_64: created template tpl-32bee19794f94962a686937a, ran python3 validate.py, confirmed envd command execution, Node.js runtime v20.20.2, public HTTP access, and node-web-sandbox validation ok.

Notes

• The test host had temporary DNS failures during the initial dependency download, so the E2B wheels were bootstrapped offline before rerunning the documented pip install -r requirements.txt command successfully.
• No Signed-off-by tag is included because DCO certification must be provided by a human submitter.

Assisted-by: Codex:GPT-5