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.

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.

src/world/rooms/hallway.md

@@ -0,0 +1,22 @@
+---
+id: hallway
+title: "[ Hallway ]"
+exitN: null
+exitS: "[[foyer]]"
+exitE: "[[cellar-stair]]"
+exitW: null
+exitU: null
+exitD: null
+items:
+  - "[[lamp]]"
+encounter: null
+---
+
+## first-visit
+A long hallway, lit by nothing. An iron oil lamp sits on a side table. The foyer is south. A stair descends east.
+
+## revisit
+The long hallway.
+
+## examined
+The hallway runs further than the house should be wide. The dust on the floor is undisturbed except where you have walked. The oil lamp is on the side table.