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/items/lamp.md

@@ -0,0 +1,10 @@
+---
+id: lamp
+names: ["lamp", "oil lamp", "torch"]
+short: "an oil lamp"
+takeable: true
+initialState:
+  lit: false
+---
+
+An iron oil lamp with a glass chimney. Currently unlit.