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>

mysteries/barricelli-files/mystery.json

@@ -8,8 +8,15 @@
       "title": "The Barricelli Files",
       "subtitle": "Scene 7 · Demonstrate OSINT skill",
       "brief": {
-        "body": "DEMONSTRATE OSINT SKILL\n\nProve that Nils Aall Barricelli was an inventor. Find a reliable source online, take a screenshot, and paste it directly onto this board. Connect the source to the authored claim with red thread, explain what the evidence proves, and submit the Case Report.",
-        "concepts": []
+        "body": "DEMONSTRATE OSINT SKILL\n\nProve that Nils Aall Barricelli was an inventor, using open sources.",
+        "checklist": [
+          "Find a reliable source online",
+          "Screenshot it and paste it onto the board",
+          "Fill in the source, date and URL so it can be verified",
+          "Connect the source to the claim with red thread",
+          "Explain what the evidence proves",
+          "Submit the Case Report"
+        ]
       },
       "claims": [
         {