Docs: Add PR contribution checklist to CONTRIBUTING.md (non-breaking)

[Chrilleweb]

Describe the change

Add a short contribution checklist to clarify expectations before opening a pull request.
This is a documentation-only change.

What to change

In CONTRIBUTING.md, add the following section immediately after the “Communication” section:

## Contribution Checklist (Before Opening a PR)

Before submitting a pull request, make sure:

- [ ] Changes are small and focused
- [ ] No breaking behavior or exit code changes
- [ ] CLI output follows existing style
- [ ] Link to related issues (if applicable)

### Tests
- [ ] Tests pass (`pnpm run test`)
- [ ] Ideally, include a test that fails without this PR but passes with it.

No other changes are required.

Scope

• Documentation only
• No code changes
• No behavior changes

Additional context

This helps contributors understand project expectations upfront and reduces review back-and-forth.