Add the user-story-conversation Pi skill following Ron Jeffries'
Card, Conversation, Confirmation mantra enhanced with Matt Wynne's
Example Mapping.
The skill guides users through six steps:
1. Card — user story
2. Conversation — Example Mapping (Rules, Examples, Questions, Answers)
3. Allium Spec — formal behavioural specification
4. fast-check Properties — executable verification
5. TypeScript ADTs — value objects and discriminated unions
6. Confirmation — verify all properties pass
Also update AGENTS.md with workflow guidance and add the new skill
to the Allium routing table. Add fast-check as a dependency.
