docs: improve scenario authoring guide

[YYH123123813]

Summary

• expands docs/scenario-spec.md with a practical scenario-authoring workflow
• adds a minimal starter template, naming guidance, author checklist, and validate/dry-run/live iteration flow
• links readers from the README validation section back to the scenario specification doc

Closes #17

AI-assisted disclosure

• Tool used: Hermes Agent
• AI-assisted parts: initial documentation drafting and revision structure
• Human review: reviewed the final wording for accuracy against the current CLI commands and repository docs
• Checks run: git diff --check