User guide with end-to-end examples

[jc-macdonald]

Write a user guide covering:

1. Quick start: define a ModelWorld + Scorer, run a grid study
2. Scoring rules reference: when to use CRPS vs WIS vs energy
3. Design patterns: screen-then-design workflow
4. Advanced: adaptive search, Annotations, multi-tier observables
5. Integration: using TradeStudy.jl with OpEngine.jl models

Include runnable examples via Literate.jl or Pluto notebooks.