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/encounters/rat.md

@@ -0,0 +1,14 @@
+---
+id: rat
+startsIn: "[[cellar-stair]]"
+initialPhase: lurking
+---
+
+## lurking
+A heavy rat watches you from the third step. Its eyes catch the light.
+
+## attack-rat-resolved
+You stamp. The rat squeals and is gone into the dark.
+
+## wait-stays
+The rat does not move. Neither do you.