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/endings/true.md

@@ -0,0 +1,7 @@
+---
+id: true
+whenFlags:
+  ratGone: true
+---
+
+You stand at the top of the stair. The thing below has settled. The door behind you opens, and outside, finally, is morning.