🧩 Issue #7: Add CI workflow for check, test, and fmt validation

Set up a GitHub Actions workflow to run on `push` and `pull_request` against `main`.

### ✅ CI should:

* Run `cargo check`
* Run `cargo test`
* Run `cargo fmt --check`
* Trigger on push + PR to `main`
* Use stable Rust (no matrix needed for now)

---

```markdown
| Field | Value |
|-------------|-------------|
| Status | In Progress |
| Type | Enhancement |
| Area | CI |
| Priority | Medium |
| Branch | ci/basic-workflow |
| Blocks | |
| Blocked By | |
| Related To | |
| Upstream | |
```

Start by locating the repository's GitHub Actions workflow location and use the issue's listed commands as the validation checklist. Done means a workflow runs on pushes and pull requests targeting main, uses stable Rust, and passes cargo check, cargo test, and cargo fmt --check.

Written by the indexing model from the issue text.