README: add CodeGuilds listing badge

[peopleworks]

SignsofAI was imported into codeguilds.dev from the MCP registry and the listing is now claimed, so link it from the distribution badge row next to the three NuGet packages.

What this changes

Why

Checklist

• dotnet test passes, with no new build warnings
• One idea per PR

If you touched the rule packs (rules.en.json / rules.es.json):

• The rule carries a suggestion — what the writer should do instead
• There's a test in tests/SignsOfAI.Core.Tests covering it
• CleanHuman in ScoringTests still scores exactly 0 (the new rule doesn't fire on ordinary prose)
• Any regex is bounded — no unbounded .*; this runs on every keystroke in the browser
• Spanish rules are derived from Spanish AI output, not translated from the English pack

If you touched the articles in Docs/Blog:

• Re-ran the CLI and updated the scores the articles quote about themselves