docs: finish public surface polish

[intelland]

Summary

• replaces the current maintainer/community contact email with intelland2024@163.com;
• removes obsolete development-stage wording from AGENTS.md;
• updates SECURITY.md from pre-publication wording to current release policy;
• removes prototype-origin commentary from the TrainCheck comparison page;
• does not change runtime behavior, API, schema, dependencies, version, README, validation evidence, release notes, workflows, tags, releases, or published 0.1.0 artifacts.

Validation

• Ruff, strict Mypy, build, Twine, and wheel/sdist metadata: PASS
• hosted Python 3.11 CI: pending