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/cellar-stair.md

@@ -0,0 +1,21 @@
+---
+id: cellar-stair
+title: "[ Cellar Stair ]"
+exitN: null
+exitS: null
+exitE: null
+exitW: "[[hallway]]"
+exitU: null
+exitD: null
+items: []
+encounter: "[[rat]]"
+---
+
+## first-visit
+The stair drops into wet stone. The hallway is west. Something at the bottom is breathing.
+
+## revisit
+The stair to the cellar.
+
+## examined
+The stairs are stone, slick with damp. You can hear water below, and something else.