docs: write CONTRIBUTING.md with fixture labelling rubric and reviewe…

[latiblack]

Closes #39

Rewrote CONTRIBUTING.md so new contributors don't have to reverse-engineer the existing fixtures to figure out what "suspicious" vs "malicious" actually means. The old one had the PR workflow and changelog format, but not the actual rubric.

What's new:

• The labelling criteria with real examples from the 11 existing entries (clean, suspicious, malicious — each with what to look for and why)
• A reviewer checklist so PRs don't bounce back and forth
• Required fields table so nothing gets forgotten in fixture PRs
• Cross-link from README

[latiblack]

PR's up: https://github.com/latiblack/grydlock-testkit/pull/new/docs/contributing-rubric

Rewrote CONTRIBUTING.md with the actual labelling rubric generalized from the 11 existing fixtures, a reviewer checklist, and a required fields table. Also cross-linked from the README.