Add a minimal GitHub Actions workflow for repository health checks

[xubinrui]

Problem

The repository currently has no automated validation for pull requests or direct pushes. Even though the project is minimal today, that means README-only or scaffold changes can merge without any repeatable check that the repository still meets a basic quality bar.

Proposal

Add a lightweight GitHub Actions workflow that runs on pull_request and push to main and validates repository health with checks that fit the current scope, for example:

• confirm required top-level files such as README.md exist
• run a markdown/doc quality check suitable for the repo's current footprint
• fail fast when future contributors introduce malformed workflow or repository metadata changes

Why this is distinct from current work

Issue #1 and PRs #2, #3, and #4 focus on repository content and scaffold changes. This issue is about engineering quality infrastructure so those and future changes can be validated automatically.

Acceptance criteria

• a CI workflow exists under .github/workflows/
• it runs automatically for pushes to main and for pull requests
• it enforces at least one real automated validation step appropriate for the repository's current size
• the README documents what the workflow checks, or the workflow is self-explanatory enough for contributors to understand its purpose