Add plain-language glossary to lower the language barrier for new contributors

[Copilot]

The repository's documentation uses heavy domain-specific jargon (fractal, substrate-agnostic, entropy, cadence, invariant, IVERO, graceful degradation, etc.) with no centralized reference, creating a steep language barrier for newcomers and non-native English speakers.

Changes

New: docs/onboarding/GLOSSARY.md

• Alphabetical glossary of 27 terms used across the system
• Each entry includes a plain-English definition, an everyday analogy callout, and relevant file/action references
• Quick Reference table at the end for fast lookups

Updated onboarding entry points

• README.md — Glossary added as step 1 in For New Contributors; added to Documentation → For Learning
• docs/onboarding/CONTRIBUTING.md — Glossary added as step 1 in Before You Contribute and in Further Reading
• docs/onboarding/MENTAL_MODEL.md — Added to Further Reading
• docs/ARCHITECTURE_OVERVIEW.md — Added to Further Reading

Example glossary entry:

### Substrate
The execution environment a workflow runs in: GitHub Actions runner, Docker
container, or local machine. Core behavior is "substrate-agnostic" — it works
the same regardless of which substrate it runs on.

**Plain English:** The platform or environment where a workflow actually runs.

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