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/letter.md

@@ -0,0 +1,8 @@
+---
+id: letter
+names: ["letter", "folded letter", "paper"]
+short: "a folded letter"
+takeable: true
+---
+
+A folded letter on yellowed paper. The hand is unfamiliar. It reads: "Come at once. The thing in the cellar is waking."