[contexto]: Tighten public types and document retrieval defaults

[sm86]

Problem

• SearchResult.items is any[]. Anyone implementing ContextoBackend ([contexto]: Implement LocalBackend #116) has to read helpers.ts to learn the shape.
• assemble() hardcodes maxResults=7, minScore=0.45, and filter={source:'summary'}. None of this is in the README or config schema — and minScore is declared in BaseConfig but never wired through.

Goal

• Type the mindmap item shape (MindmapItem / ScoredMindmapItem).
• Honor maxResults, minScore, retrievalFilter from config.
• Document the defaults in the package README and docs/contexto.md.