Add authorable per-level briefing checklist

The case briefing showed a wall of body text. Add a checklist of short,
player-tickable steps authored per level: a new `checklist` on the level brief
(table osint.level_brief_checklist_items), read/written with the board, cloned
on template freeze/instantiate, and carried through the mystery importer and the
mystery:pull exporter. Player tick state is guidance-only, persisted locally per
level rather than on the shared board. Convert the Barricelli inventor-proof
brief from prose into a short intro plus a six-step checklist.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/exportMystery.ts

@@ -75,8 +75,8 @@ function serializeLevel(board: TemplateBoardExport, assetFilenames: Map<string,
     title: state.title,
     subtitle: state.subtitle || undefined,
     timelineRange: timeline?.range,
-    brief: state.brief.body || state.brief.concepts.length
-      ? { body: state.brief.body, concepts: state.brief.concepts.map(concept => ({ label: concept.label, context: concept.context, expectedPartyKind: concept.expectedPartyKind || 'person' })) }
+    brief: state.brief.body || state.brief.concepts.length || state.brief.checklist?.length
+      ? prune({ body: state.brief.body, concepts: state.brief.concepts.map(concept => ({ label: concept.label, context: concept.context, expectedPartyKind: concept.expectedPartyKind || 'person' })), checklist: state.brief.checklist })
       : undefined,
     documents: documents.map(document => prune({
       key: documentKeyById.get(document.id)!, title: document.title, fileType: document.fileType, captureKind: document.captureKind,