Write a scenario-authoring guide (choosing values, aliases, forbidden vs appropriate)

[bamdadd]

Context

Adding a scenario is the most common contribution, but the rules for a good one are scattered across docs/DESIGN.md, the scenarios.py docstrings, and the type docstrings. A short guide would lower the barrier: how to pick a meaningful-but-unique value, when to add aliases vs confirm_phrases, the deny-list semantics of forbidden (unlisted = allowed), and how appropriate_flows drives utility.

Acceptance criteria

• A concise guide (e.g. docs/SCENARIOS.md or a section appended to docs/DESIGN.md) covering: value selection, aliases, forbidden vs appropriate semantics, and registering in ALL_SCENARIOS.
• Uses only synthetic, domain-neutral placeholder examples — no real data.
• States the invariants a scenario must satisfy (unique attribute names/recipient ids; forbidden/appropriate reference real ids).
• Linked from CONTRIBUTING.md and/or README.md.