Make mystery importer seed notes and tolerate sparse manifests

Add a notes[] array to the manifest (note exhibits with presentation),
so the Scene 6 phone-note board is reproducible from source instead of
hand-authored. Also default documents/folders/brief/subtitle so
narrative-only mysteries (e.g. barricelli-files) import cleanly.

Captures mysteries/barricelli-phone-note/mystery.json from the authored
template (note: "PHONE FOR GLITCH HUNTER / Call: 5550100").

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

mysteries/barricelli-phone-note/mystery.json

@@ -0,0 +1,20 @@
+{
+  "slug": "barricelli-phone-note",
+  "name": "Phone note",
+  "title": "Phone note",
+  "subtitle": "",
+  "brief": { "body": "", "concepts": [] },
+  "documents": [],
+  "folders": [],
+  "notes": [
+    {
+      "title": "Note",
+      "content": "PHONE FOR GLITCH HUNTER\n\nCall: 5550100",
+      "presentation": "luggage",
+      "x": 420,
+      "y": 260,
+      "width": 230,
+      "height": 180
+    }
+  ]
+}