feat(mystery): migration script and produced markdown content

Adds scripts/migrate-mystery-content.ts which reads rooms, items,
encounters, and endings from TypeScript source and emits byte-identical
markdown files under src/mystery/world/{rooms,items,encounters,endings}/.
Installs tsx as a devDep to support .ts imports across src/ during the
one-shot run.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/world/rooms/foyer.md

@@ -0,0 +1,23 @@
+---
+id: foyer
+title: "[ Foyer ]"
+exitN: "[[hallway]]"
+exitS: null
+exitE: null
+exitW: null
+exitU: null
+exitD: null
+items:
+  - "[[letter]]"
+encounter: null
+safe: true
+---
+
+## first-visit
+You stand in the foyer of a house you do not remember entering. The door behind you has closed without sound. A folded letter lies on a small table. A hallway leads north.
+
+## revisit
+The foyer. The door behind you is still closed.
+
+## examined
+A foyer with peeling paper. A small table holds nothing but the letter. The air smells of cold stone. A hallway leads north.