checkstyle.rs:6, gitlab_quality.rs:6: bare URLs in doc comments should use <> brackets — clippy::bare_url

[EffortlessSteven]

Clippy warning: bare URLs in doc comments should be wrapped in <> or made proper Markdown links.

Locations:

• `crates/diffguard-core/src/checkstyle.rs:6`: `https://checkstyle.org/index.html\`
• `crates/diffguard-core/src/gitlab_quality.rs:6`: `https://docs.gitlab.com/ee/ci/testing/code_quality.html#implement-a-custom-tool\`

Fix: Wrap bare URLs in angle brackets per Rust doc convention:

/// Schema reference: <https://checkstyle.org/index.html>

```
warning: you should put bare URLs between `<`/`>` or make a proper Markdown link
--> crates/diffguard-core/src/checkstyle.rs:6:23
|
6 | //! Schema reference: https://checkstyle.org/index.html
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `https://checkstyle.org/index.html`
```